ALBERT

Description: This investigation analyzes the effect of vortex wakes on the Lagrangian displacement of particles induced by the passage of an obstacle in a two-dimensional incompressible and inviscid fluid. In addition to the trajectories of individual particles, we also study their drift and the corresponding total drift areas in the Föppl and Kirchhoff potential flow models. Our findings, which are obtained numerically and in some regimes are also supported by asymptotic analysis, are compared to the wakeless potential flow which serves as a reference. We show that in the presence of the Föppl vortex wake, some of the particles follow more complicated trajectories featuring a second loop. The appearance of an additional stagnation point in the Föppl flow is identified as a source of this effect. It is also demonstrated that, while the total drift area increases with the size of the wake for large vortex strengths, it is actually decreased for small circulation values. On the other hand, the Kirchhoff flow model is shown to have an unbounded total drift area. By providing a systematic account of the wake effects on the drift, the results of this study will allow for more accurate modeling of hydrodynamic stirring.

Description: The slow motion of a circular cylinder in a plane Poiseuille flow in a microchannel is analyzed for a wide range of cylinder radii and positions across the channel. The cylinder translates parallel to the channel walls and rotates about its axis. The Stokes approximation is used and the problem is solved analytically using the Papkovich-Fadle eigenfunction expansion and the least-squares method. The stream function and the pressure distribution of the flow field are obtained as results. The force and moment exerted on the cylinder, and the pressure change far from the cylinder, are calculated and shown as functions of the size and location of the cylinder. The results confirm some reciprocal relations exactly. In particular, the translational and rotational velocities of the drifting cylinder in the existing Poiseuille flow are determined. The induced pressure change, when the cylinder drifts in the Poiseuille flow, is also calculated. Some typical streamline patterns, depending on the size and location of the cylinder, are shown and discussed. When the cylinder translates and/or rotates in the channel blocked at infinity, a series of Moffatt eddies appears far from the cylinder in the channel, as expected.

Description: Interactions between capillary and elastic effects are relevant to a variety of applications from micro- and nano-scale manufacturing to biological systems. In this work, we investigate capillary flows in flexible, millimeter-scale cylindrical elastic tubes. We demonstrate that surface tension can cause sufficiently flexible tubes to collapse and coalesce spontaneously through non-axisymmetric buckling, and develop criteria for the initial deformation and complete collapse of a circular tube. Experimental results for capillary rise and evaporation of a liquid in a flexible tube are presented, and several regimes are seen for the equilibrium state of a flexible tube deforming under capillary pressure. Deformations of the tube walls are measured in different regimes and compared with a shell theory model. Analysis and experimental results show that despite the complex and non-axisymmetric deformed shapes of cylindrical structures, the elastocapillary length used in previous literature for flat plates and sheets can be used to predict the behavior of flexible tubes.

Description: We present laboratory experimental results demonstrating that librational forcing of an ellipsoidal container of water can produce intense motions through the mechanism of a libration driven elliptical instability (LDEI). These libration studies are conducted using an ellipsoidal acrylic container filled with water. A particle image velocimetry method is used to measure the 2D velocity field in the equatorial plane over hundreds libration cycles for a fixed Ekman number, E = 2 × 10 −5 . In doing so, we recover the libration induced base flow and a time averaged zonal flow. Further, we show that LDEI in non-axisymmetric container geometries is capable of driving both intermittent and saturated turbulent motions in the bulk fluid. Additionally, we measure the growth rate and amplitude of the LDEI induced excited flow in a fully ellipsoidal container at more extreme parameters than previously studied [Noir et al. , “Experimental study of libration-driven flows in nonaxisymmetric containers,” Phys. Earth Planet. Inter. 204-205 , 1 (2012); Cébron et al. , Phys. Fluids 24 , 061703, “Libration driven elliptical instability,” (2012)]. Excitation of bulk filling turbulence by librational forcing provides a mechanism for transferring rotational energy into turbulent fluid motion and thus can play an important role in the thermal evolution, interior dynamics, and magneto-hydrodynamics of librating bodies, as appear to be common in solar system settings [e.g., Comstock and Bills, “A solar system survey of forced librations in longitude,” J. Geophys. Res. Planets 108 , 1 (2003)].

Description: We numerically study the displacement flow of two iso-viscous Newtonian fluids in an inclined two-dimensional channel, formed by two parallel plates. The results are complementary to our previous studies on displacement flows in pipes and channels. The heavier displacing fluid moves the lighter displaced fluid in the downward direction. Three dimensionless groups largely describe these flows: the densimetric Froude number ( Fr ), the Reynolds number ( Re ), and the duct inclination (β). As a first order approximation, we are able to classify different flow regimes phenomenologically in a two-dimensional ( Fr ; Re cosβ/ Fr )-plane and provide leading order expressions for the transitions between different regimes. The stabilizing and/or de-stabilizing effects of the imposed mean flow on buoyant exchange flows (zero imposed velocity) are described for a broad range of dimensionless parameters.

Description: Compressible granular materials are involved in many applications, some of them being related to energetic porous media. Gas permeation effects are important during their compaction stage, as well as their eventual chemical decomposition. Also, many situations involve porous media separated from pure fluids through two-phase interfaces. It is thus important to develop theoretical and numerical formulations to deal with granular materials in the presence of both two-phase interfaces and gas permeation effects. Similar topic was addressed for fluid mixtures and interfaces with the Discrete Equations Method (DEM) [R. Abgrall and R. Saurel, “Discrete equations for physical and numerical compressible multiphase mixtures,” J. Comput. Phys. 186 (2), 361-396 (2003)] but it seemed impossible to extend this approach to granular media as intergranular stress [K. K. Kuo, V. Yang, and B. B. Moore, “Intragranular stress, particle-wall friction and speed of sound in granular propellant beds,” J. Ballist. 4 (1), 697-730 (1980)] and associated configuration energy [J. B. Bdzil, R. Menikoff, S. F. Son, A. K. Kapila, and D. S. Stewart, “Two-phase modeling of deflagration-to-detonation transition in granular materials: A critical examination of modeling issues,” Phys. Fluids 11 , 378 (1999)] were present with significant effects. An approach to deal with fluid-porous media interfaces was derived in Saurel et al. [“Modelling dynamic and irreversible powder compaction,” J. Fluid Mech. 664 , 348-396 (2010)] but its validity was restricted to weak velocity disequilibrium only. Thanks to a deeper analysis, the DEM is successfully extended to granular media modelling in the present paper. It results in an enhanced version of the Baer and Nunziato [“A two-phase mixture theory for the deflagration-to-detonation transition (DDT) in reactive granular materials,” Int. J. Multiphase Flow 12 (6), 861-889 (1986)] model as symmetry of the formulation is now preserved. Several computational examples are shown to validate and illustrate method’s capabilities.

Description: We perform a theoretical and numerical study of the Coulomb-driven electroconvection flow of a dielectric liquid between two coaxial cylinders. The specific case, where the inner to outer diameter ratio is 0.5, is analyzed. A strong unipolar injection of ions either from the inner or outer cylinder is considered to introduce free charge carriers into the system. A finite volume method is used to solve all governing equations including Navier-Stokes equations and a simplified set of Maxwell’s equations. The flow is characterized by a subcritical bifurcation in the finite amplitude regime. A linear stability criterion and a nonlinear one that correspond to the onset and stop of the flow motion, respectively, are linked with a hysteresis loop. In addition, we also explore the behavior of the system for higher values of the stability parameter. For inner injection, we observe a transition between the patterns made of 7 and 8 cells, before an oscillatory regime is attained. Such a transition leads to a second finite amplitude stability criterion. A simple modal analysis reveals that the competition of different modes is at the origin of this behavior. The charge density, as well as velocity field distributions is provided to help understand the bifurcation behavior.

Description: Phytoplankton patchiness, namely the heterogeneous distribution of microalgae over multiple spatial scales, dramatically impacts marine ecology. A spectacular example of such heterogeneity occurs in thin phytoplankton layers (TPLs), where large numbers of photosynthetic microorganisms are found within a small depth interval. Some species of motile phytoplankton can form TPLs by gyrotactic trapping due to the interplay of their particular swimming style (directed motion biased against gravity) and the transport by a flow with shear along the direction of gravity. Here we consider gyrotactic swimmers in numerical simulations of the Kolmogorov shear flow, both in laminar and turbulent regimes. In the laminar case, we show that the swimmer motion is integrable and the formation of TPLs can be fully characterized by means of dynamical systems tools. We then study the effects of rotational Brownian motion or turbulent fluctuations (appearing when the Reynolds number is large enough) on TPLs. In both cases, we show that TPLs become transient, and we characterize their persistence.

Description: In this study, rock friction ‘stick-slip’ experiments are used to develop constraints on models of earthquake recurrence. Constant-rate loading of bare rock surfaces in high quality experiments produces stick-slip recurrence that is periodic at least to second order. When the loading rate is varied, recurrence is approximately inversely proportional to loading rate. These laboratory events initiate due to a slip rate-dependent process that also determines the size of the stress drop and as a consequence, stress drop varies weakly but systematically with loading rate. This is especially evident in experiments where the loading rate is changed by orders of magnitude, as is thought to be the loading condition of naturally occurring, small repeating earthquakes driven by afterslip, or low-frequency earthquakes loaded by episodic slip. As follows from the previous studies referred to above, experimentally observed stress drops are well described by a logarithmic dependence on recurrence interval that can be cast as a non-linear slip-predictable model. The fault's rate dependence of strength is the key physical parameter. Additionally, even at constant loading rate the most reproducible laboratory recurrence is not exactly periodic, unlike existing friction recurrence models. We present example laboratory catalogs that document the variance and show that in large catalogs, even at constant loading rate, stress drop and recurrence co-vary systematically. The origin of this covariance is largely consistent with variability of the dependence of fault strength on slip rate. Laboratory catalogs show aspects of both slip and time predictability and successive stress drops are strongly correlated indicating a ‘memory’ of prior slip history that extends over at least one recurrence cycle.

Description: The model of gas bubble growth in high-viscous gas-saturated magmatic melt, subjected to rapid decompression, is presented in the current study. It is shown that consideration of unsteady character of the process is extremely important in a wide range of supersaturation. The analytical solution is found for the profile of dissolved gas concentration and the rate of bubble growth. The model of kinetics of overall degassing is developed. This model is based on distinguishing the so-called “forbidden” zone in the melt volume with suppressed formation of the new nucleation sites. The simple analytical dependences of the number of nucleating bubbles and typical nucleation time on the value of initial decompression were derived together with time dependence of volumetric concentration of the gas phase. Our results match the available experimental data.

Description: We investigate the effect of viscosity contrast on the stability of gravitationally unstable, diffusive layers in porous media. Our analysis helps evaluate experimental observations of various diffusive (boundary) layer models that are commonly used to study the sequestration of CO 2 in brine aquifers. We evaluate the effect of viscosity contrast for two basic models that are characterized with respect to whether or not the interface between CO 2 and brine is allowed to move. We find that diffusive layers are in general more unstable when viscosity decreases with depth within the layer compared to when viscosity increases with depth. This behavior is in contrast to the one associated with the classical displacement problem of gravitationally unstable diffusive layers that are subject to mean flow. For the classical problem, a greater instability is associated with the displacement of a more viscous, lighter fluid along the direction of gravity by a less viscous, heavier fluid. We show that the contrasting behavior highlighted in this study is a special case of the classical displacement problem that depends on the relative strength of the displacement and buoyancy velocities. We demonstrate the existence of a critical viscosity ratio that determines whether the flow is buoyancy dominated or displacement dominated. We explain the new behaviors in terms of the interaction of vorticity components related to gravitational and viscous effects.

Description: The development of a round liquid jet under the influence of a confined coaxial flow of an immiscible liquid of comparable density (central to annular flow density ratio of 8:10) was investigated in the vicinity of the nozzle exit. Two flow regimes were considered; one where the annular flow is faster than the central jet, so the central liquid jet is accelerated and one where the annular flow is slower, so the central liquid jet is decelerated. The central jet was visualised by high speed photography. Three modes of jet development were identified and classified in terms of the Reynolds number, Re, of the central jet which was in the range of 525 〈 Re 〈 2725, a modified definition of the Weber number, We, which allows the distinction between accelerating and deceleration flows and was in the range of −22 〈 We 〈 67 and the annular to central Momentum Ratio, MR, of the two streams which was in the range of 3.6 〈 MR 〈 91. By processing the time resolved jet images using Proper Orthogonal Decomposition (POD), it was possible to reduce the description of jet morphology to a small number of spatial modes, which isolated the most significant morphologies of the jet development. In this way, the temporal and spatial characteristics of the instabilities on the interface were clearly identified which highlights the advantages of POD over direct observation of the images. Relationships between the flow parameters and the interfacial waves were established. The wavelength of the interfacial instability was found to depend on the velocity of the fastest moving stream, which is contrary to findings for fluids with large density differences.

Description: Interaction of a vortex ring impinging on multiple permeable screens orthogonal to the ring axis was studied to experimentally investigate the persistence and decay of vortical structures inside the screen array using digital particle image velocimetry in a refractive index matched environment. The permeable screens had porosities (open area ratios) of 83.8%, 69.0%, and 55.7% and were held by a transparent frame that allowed the screen spacing to be changed. Vortex rings were generated using a piston-cylinder mechanism at nominal jet Reynolds numbers of 1000, 2000, and 3000 with piston stroke length-to-diameter ratios of 2 and 3. The interaction of vortex rings with the porous medium showed a strong dependence of the overall flow evolution on the screen porosity, with a central flow being preserved and vortex ring-like structures (with smaller diameter than the primary vortex ring) being generated near the centerline. Due to the large rod size used in the screens, immediate reformation of the transmitted vortex ring with size comparable to the primary ring (as has been observed with thin screens) was not observed in most cases. Since the screens have lower complexity and high open area ratios, centerline vortex ring-like flow structures formed with comparable size to the screen pore size and penetrated through the screens. In the case of low porosity screens (55.7%) with large screen spacing, re-emergence of large scale (large separation), weak vortical structures/pairs (analogous to a transmitted vortex ring) was observed downstream of the first screen. Additional smaller scale vortical structures were generated by the interaction of the vortex ring with subsequent screens. The size distribution of the generated vortical structures were shown to be strongly affected by porosity, with smaller vortical structures playing a stronger role as porosity decreased. Finally, porosity significantly affected the decay of total energy, but the effect of screen spacing decreased as porosity decreased.

Description: Vortex cavitation forming in the leading-edge vortices of a delta wing was examined to determine how the individual cavitation bubbles incepted, grew, interacted with the underlying vortical flow and produced acoustic tones. The non-cavitating vortical flow over the delta wing was chosen to be similar to those previously reported in the literature. It was found that vortex breakdown was unaffected by the presence of incipient and developed vortex cavitation bubbles in the vortex core. While some cavitation bubbles incepted, grew, and collapsed relatively quickly, others reached an equilibrium position wherein the bubble tip was stationary in the laboratory frame at a particular location along the vortex axis. For a given attack angle, the equilibrium location moved upstream with a reduction in free stream cavitation number. It is shown that the existence of these stationary vortex bubbles is possible when there is a balance between the axial growth of the bubble along the vortex axis and the opposite motion of the axial jetting flow in the vortex core, and only a single equilibrium position is possible along the axially evolving vortex for a given free stream cavitation number. These transient and stationary vortex bubbles emit significant cavitation noise upon inception, growth, and collapse. The spectral content of the noise produced was expected to be related to the interaction of the bubble with the surrounding vortical flow in a manner similar to that reported in previous studies, where sustained tones were similar to the underlying vortex frequency. However, in the present study, the dominant frequency and higher harmonics of the tones occur at a higher frequency than that of the underlying vortex. Hence, it is likely that the highly elongated stationary bubbles have higher-order volume oscillations compared to the two-dimensional radial mode of the vortex cores of vortex cavitation bubbles with much smaller diameter-to-length ratios.

Description: We derive equations for HTI and orthorhombic symmetries to analyze fluid substitution effects in porous fractured media. The derivations are based on the anisotropic Gassmann equation and linear slip theory. We assess the influence of fluid substitution (gas, brine, and oil), on elastic moduli, velocities, anisotropy, and azimuthal amplitude variations. We find that in the direction normal to fractures, P-wave moduli increase as much as 56% and P-wave velocity increases up to 19% for gas-to-brine substitution. For the direction parallel to fractures, P-wave velocity remains almost constant when porosity is low (5%), but can increase up to 4% if porosity is high (25%). Since P-waves in two different directions have different sensitivities to fluids and fractures, the Thomsen's parameters (defined for HTI and orthorhombic symmetries), ε and δ , are sensitive to fluid types and fractures. We also found that δ is sensitive to porosity for liquid saturation, but insensitive to porosity for the case of gas saturation. Gassmann assumes (and as has been observed) that shear modulus does not depend on fluids. And we observe no changes in shear-wave splitting ( γ ) for different fluids. The azimuthal amplitude variation is dependent on fluid types, fractures and porosity. We observe up to 12% increase in azimuthal amplitude variation for low porosity gas sands after brine saturation, and 6% decrease for high porosity gas sands. We find that the percentage changes in gas-to-oil substitution are about half that of the gas-to-brine case. The equations we have derived provide a useful tool to quantitatively evaluate the effects of fluid substitution on seismic anisotropy.

Description: In the central part of Fennoscandia the crust is currently rising, because of the delayed response of the viscous mantle to melting of the Late Pleistocene ice sheet. This process, called Glacial Isostatic Adjustment (GIA), causes a negative anomaly in the present-day static gravity field as isostatic equilibrium has not been reached yet. Several studies have tried to use this anomaly as a constraint on models of GIA, but the uncertainty in crustal and upper mantle structures has not been fully taken intoaccount. Therefore, our aim is to revisit this using improved crustal models and compensation techniques. We find that, in contrast with other studies, the effect of crustal anomalies on the gravity field cannot be effectively removed, because of uncertainties in the crustal and upper mantle density models. Our second aim is to estimate the effects on geophysical models, which assume isostatic equilibrium, after correcting the observed gravity field with numerical models for GIA. We show that correcting for GIA in geophysical modelling can give changes of several km in the thickness of structural layers of modeled lithosphere, which is a small but significant correction. Correcting the gravity field for GIA prior to assuming isostatic equilibrium and inferring density anomalies might be relevant in other areas with ongoing post-glacial rebound such as North America and the polar regions.

Description: Large variability of earthquake stress drops and scaled energy has been commonly reported in the literature, but it is difficult to assess how much of this variability is caused by underlying physical source processes rather than simply observational uncertainties. Here, we examine a variety of dynamically realistic rupture scenarios for circular and elliptical faults and investigate to what extent the variability in seismically estimated stress drops and scaled energy comes from differences in source geometry, rupture directivity, and rupture speeds. We numerically simulate earthquake source scenarios using a cohesive-zone model with the small-scale yielding limit, where the solution approaches a singular crack model with spontaneous healing of slip. Compared to symmetrical circular source models, asymmetrical models result in larger variability of estimated corner frequencies and scaled energy over the focal sphere. The general behavior of the spherical averages of corner frequencies and scaled energy in the subshear regime extends to the supershear regime, although shear Mach waves generated by the propagation of supershear rupture lead to much higher corner-frequency and scaled-energy estimates locally. Our results suggest that at least a factor of two difference in the spherical average of corner frequencies is expected in observational studies simply from variability in source characteristics almost independent of the actual stress drops, translating into a factor ofeight difference in estimated stress drops. Furthermore, radiation efficiency estimates derived from observed seismic spectra should not be directly interpreted as describing rupture properties unless there are independent constraints on rupture speed and geometry.

Description: Laminar flow over a periodic array of cylindrical surface roughness elements is simulated with an immersed boundary spectral method both to validate the method for subsequent studies and to examine how persistent streamwise vortices are introduced by a low Reynolds number roughness element. Direct comparisons are made with prior studies at a roughness-based Reynolds number Re k (= U ( k ) k / ν ) of 205 and a diameter to spanwise spacing ratio d / λ of 1/3. Downstream velocity contours match present and past experiments very well. The shear layer developed over the top of the roughness element produces the downstream velocity deficit. Upstream of the roughness element, the vortex topology is found to be consistent with juncture flow experiments, creating three cores along the recirculation line. Streamtraces stemming from these upstream cores, however, have unexpectedly little effect on the downstream flowfield as lateral divergence of the boundary layer quickly dissipates their vorticity. Long physical relaxation time of the recirculating wake behind the roughness remains a prominent issue for simulating this type of flowfield.

Description: Petrophysical properties of rocks and their applicability at larger scale are a challenging topic in Earth sciences. Petrophysical properties of rocks are severely affected by: boundary conditions, rock fabric/microstructure, and tectonics that require a multi-scale approach to be properly defined. Here we: (1) report laboratory measurements of density, porosity, permeability and P-wave velocities at increasing confining pressure conducted on Miocene foredeep sandstones (Frosinone Fm.); (2) compare the laboratory results with larger-scale geophysical investigations; (3) discuss the effect of thrusting on the properties of sandstones. At ambient pressure, laboratory porosity varied from 2.2% to 13.8% and P-wave velocities (Vp) from 1.5 km/s to 2.7 km/s. The P-wave velocity increased with confining pressure, reaching between 3.3 km/s to 4.7 km/s at 100 MPa. In situ Vp profiles, measured using sonic logs, matched the ultrasonic laboratory measurement well. The permeability varied between 1.4 × 10 -15  m 2 to 3.9 × 10 -15  m 2 and was positively correlated with porosity. The porosity and permeability of samples taken at various distances to the Olevano-Antrodoco fault plane progressively decreased with distance while P-wave velocity increased. At about 1 km from the fault plane, the relative variations reached 43%, 65% and 20% for porosity, permeability and P-wave velocity, respectively. This suggests that tectonic loading changed the petrophysical properties inherited from sedimentation and diagenesis. Using field constraints and assuming overburden-related inelastic compaction in the proximity of the fault plane, we conclude that the fault reached the mechanical condition for rupture in compression at differential stress of 64.8 MPa at a depth of 1500 m.

Description: Solutal Marangoni instability (SMI) is investigated both in 2D and 3D using a combined Cahn-Hilliard and Navier-Stokes model in a finite system. Fe-Sn is chosen as a representative alloy system since the phase diagram reveals a region with a miscibility gap, where two liquid phases, namely, the Fe-rich phase L 1 and the Sn-rich phase L 2 , are in chemical equilibrium. In 3D, considering a perturbed liquid cylinder ( L 2 phase) with a length of λ and a radius of R 0 embedded in the middle of a simulation box of λ × H × H (length × width × height) surrounded by the phase L 1 , we find that the perturbation induced Marangoni flow is either clockwise or anti-clockwise depending on the mean curvature difference between the convex and concave regions which is affected by the ratio of λ/ R 0 . The critical ratio of λ/ R 0 for SMI is shown to be invariant for different Marangoni numbers as well as independent of the geometrical properties of the L 1 phase. In 2D, a perturbed liquid pipe with a length of λ and a radius of R 0 embedded in the middle of a simulation box of λ × H (length × height) is taken into account. Due to different curvature constitution, the critical ratio of λ/ R 0 for SMI depends on the height of the L 1 phase.

Description: For several years, a promising Plasma Synthetic Jet actuator for high-speed flow control has been under development at ONERA. So far, its confined geometry and small space-time scales at play have prevented its full experimental characterization. Complementary accurate numerical simulations are then considered in this study in order to provide a complete aerothermodynamic description of the actuator. Two major obstacles have to be overcome with this approach: the modeling of the energy deposited by the electric arc and the accurate computation of the transient response of the cavity generating the pulsed jet. To solve the first problem, an Euler solver coupled with an electric circuit model was used to evaluate the energy deposition in the cavity. Such a coupling is performed by considering the electric field between the two electrodes. The second issue was then addressed by injecting these source terms in large Eddy simulations of the entire actuator. Aerodynamic results were finally compared with Schlieren visualizations. Using the proposed methodology, the temporal evolution of the jet front is remarkably well predicted.

Description: In a subduction system the force and the energy required to deform the overriding plate are generally thought to come from the negative buoyancy of the subducted slab and its potential energy, respectively. Such deformation might involve extension and backarc basin formation, or shortening and mountain building. How much of the slab's potential energy is consumed during overriding plate deformation remains unknown. In this work, we present dynamic three-dimensional laboratory experiments of progressive subduction with an overriding plate to quantify the force ( F OPD ) that drives overriding plate deformation and the associated energy dissipation rate (Φ OPD ), and we compare them with the negative buoyancy ( F BU ) of the subducted slab and its total potential energy release rate (Φ BU ), respectively. We varied the viscosity ratio between the plates and the sub-lithospheric upper mantle with η SP /η UM  = 157-560, and the thickness of the overriding plate with T OP  = 0.5-2.5 cm (scaling to 25-125 km in nature). The results show that F OPD / F BU has average values of 0.5-2.0%, with a maximum of 5.3% and Φ OPD /Φ BU has average values of 0.05-0.30%, with a maximum of 0.41%. The results indicate that only a small portion of the negative buoyancy of the slab and its potential energy are used to deform the overriding plate. Our models also suggest that the force required to deform the overriding plate is of comparable magnitude to the ridge push force. Furthermore, we show that in subduction models with an overriding plate bending dissipation at the subduction zone hinge remains low (3-15% during steady-state subduction).

Description: We investigated the influence of earthquake source complexity on the extent of inundation caused by the resulting tsunami. We simulated 100 scenarios with collocated sources of variable slip on the Hikurangi subduction-interface in the vicinity of Hawke's Bay and Poverty Bay in New Zealand and investigated the tsunami effects on the cities of Napier and Gisborne. Rupture complexity was found to have a first order effect on flow depth and inundation extent for local tsunami sources. The position of individual asperities in the slip distribution on the rupture interface control to some extent how severe inundation will be. However, predicting inundation extent in detail from investigating the distribution of slip on the rupture interface proves difficult. Assuming uniform slip on the rupture interface in tsunami models can underestimate the potential impact and extent of inundation. For example, simulation of an M W 8.7 to M W 8.8 earthquake with uniform slip reproduced the area that could potentially be inundated by equivalent non-uniform slip events of M W 8.4. De-aggregation, to establish the contribution of different sources with different slip distributions to the probabilistic hazard, cannot be performed based on magnitude considerations alone. We propose two predictors for inundation severity based on the offshore tsunami wave field using the linear wave equations in an attempt to keep costly simulations of full inundation to a minimum.

Description: Teleseismic transfer function analysis is used to investigate crust and upper mantle velocity structure in the vicinity of the active eastern Tennessee seismic zone (ETSZ). The ETSZ is associated with the New York – Alabama (NY-AL) magnetic lineament, a prominent aeromagnetic anomaly indicative of Grenville-age, basement structure. Radial component, P-wave transfer functions for ten short-period stations operated by the Center for Earthquake Research and Information (CERI) are inverted for velocity structure. Velocity profiles are also determined for three broadband stations by converting the instrument response to that of an S-13 short-period seismometer. Distinct differences in the velocity profiles are found for stations located on either side of the NY-AL magnetic lineament; velocities west of the lineament are lower than velocities to the east of the lineament in the upper 10 km and in the depth range 30 to 50 km. A gradational Moho boundary is found beneath several stations located in the Valley and Ridge province. A Moho boundary is absent at four Valley and Ridge stations located east of the magnetic lineament and south of 35.5°N.

Description: This integrated study provides significant insight into parameters controlling the acoustic and reservoir properties of microporous limestones, improving the knowledge of the relationships among petrophysic and microstructural content. Petrophysical properties measured from laboratory and logging tools (porosity, permeability, electrical conductivity and acoustic properties) have been coupled with thin section and SEM observations on the EST205 borehole from the Oxfordian limestone aquifer of the Eastern part of the Paris Basin. A major achievement is the establishment of the link between micrite microtexture types (particle morphology and nature of inter-crystal contacts) and the physical response, introducing a new effective and interesting rock-typing approach for microporous reservoirs. Fluid-flow properties are enhanced by the progressive augmentation of intercrystalline microporosity and associated pore throat diameter, as the coalescence of micrite particles decreases. Concerning acoustic properties, the slow increase of P-wave velocity can be seen as a reflection of crystal size and growing contact cementation leading to a more cohesive and stiffer micrite microtexture. By applying poroelasticity theory on our samples, we show that velocity dispersion can be a very useful tool for data discrimination in carbonates. This dispersion analysis highlights the presence of microcracks in the rocks, and their overall effect on acoustic and transport properties. The presence of microcracks is also confirmed with observations and permeability measurements under high confining pressure. Finally, a possible origin of high porous levels in neritic limestones is a mineralogical transformation of carbonates through freshwater-related diagenesis during subaerial exposure time. Finally, by applying poroelasticity theory on our samples, we show that velocity dispersion can be a very useful tool for data discrimination in carbonates.

Description: The “local porosity theory” proposed by Hilfer was revisited to develop a “local clay theory” (LCT) that establishes a quantitative relationship between the effective electrical conductivity and clay distribution in clay-rocks. This theory is primarily based on a “local simplicity” assumption; under this assumption, the complexity of spatial clay distribution can be captured by two local functions, namely, the local clay distribution and the local percolation probability, which are calculated from a partitioning of a mineral map. The local clay distribution provides information about spatial clay fluctuations and the local percolation probability describes the spatial fluctuations in the clay connectivity. This LCT was applied to (a) a mineral map made from a Callovo-Oxfordian mudstone sample and (b) (macroscopic) electrical conductivity measurements performed on the same sample. The direct and inverse modeling show two results. First, the textural and classical model assuming that the electrical anisotropy of clay-rock is mainly controlled by the anisotropy of the sole clay matrix provides inconsistent inverted values. An other textural effect, the anisotropy induced by elongated and oriented non-clayey grains, should be considered. Second, the effective conductivity values depend primarily on the choice of the inclusion-based models used in the LCT. The impact of local fluctuations of clay content and connectivity on the calculated effective conductivity is lower.

Description: Volcanic ash is often deposited in a hot state. Volcanic ash containing glass, deposited above the glass transition interval, has the potential to sinter viscously both to itself (particle-particle) and to exposed surfaces. Here, we constrain the kinetics of this process experimentally under non-isothermal conditions using standard glasses. In the absence of external load, this process is dominantly driven by surface relaxation. In such cases the sintering process is rate-limited by the melt viscosity, the size of the particles and the melt-vapour interfacial tension. We propose a polydisperse continuum model that describes the transition from a packing of particles to a dense pore-free melt and evaluate its efficacy in describing the kinetics of volcanic viscous sintering. We apply our model to viscous sintering scenarios for cooling crystal-poor rhyolitic ash using the 2008 eruption of Chaitén volcano as a case example. We predict that moderate linear cooling rates of 〉10 -1  °C.min -1 can result in the common observation of incomplete sintering and the preservation of pore networks.

Description: Snapshot and classical proper orthogonal decomposition (POD) are used to examine the large-scale, energetic motions in fully developed turbulent pipe flow at Re D = 47,000 and 93,000. The snapshot POD modes come in pairs, representing the same azimuthal mode number but with a simple phase shift. The first 10 snapshot POD modes, associated with the very large scale motions (VLSMs), contribute 43% of the average Reynolds shear stress, and for first 80 modes u ′ and v ′ are anti-correlated so that they all contribute to positive shear stress events. The attached motions are contained in the lower order modes, and detached motions do not appear until snapshot POD mode numbers ≥15. We find that snapshot POD can introduce mode mixing, which is avoided in classical POD. Classical POD also gives frequency information, confirming that the low order modes capture well the behavior of the very large scale motions.

Description: We explore the instabilities developed in a fluid in which viscosity depends on temperature. In particular, we consider a dependency that models a very viscous (and thus rather rigid) lithosphere over a convecting mantle. To this end, we study a 2D convection problem in which viscosity depends on temperature by abruptly changing its value by a factor of 400 within a narrow temperature gap. We conduct a study which combines bifurcation analysis and time-dependent simulations. Solutions such as limit cycles are found that are fundamentally related to the presence of symmetry. Spontaneous plate-like behaviors that rapidly evolve towards a stagnant lid regime emerge sporadically through abrupt bursts during these cycles. The plate-like evolution alternates motions towards either the right or the left, thereby introducing temporary asymmetries on the convecting styles. Further time-dependent regimes with stagnant and plate-like lids are found and described.

Description: We study the shock wave structure in a rarefied polyatomic gas based on a simplified model of extended thermodynamics in which the dissipation is due only to the dynamic pressure. In this case the differential system is very simple because it is a variant of Euler system with a new scalar equation for the dynamic pressure [T. Arima, S. Taniguchi, T. Ruggeri, and M. Sugiyama, Phys. Lett. A376, 2799–2803 (2012)]. It is shown that this theory is able to describe the three types of the shock wave structure observed in experiments: the nearly symmetric shock wave structure (Type A, small Mach number), the asymmetric structure (Type B, moderate Mach number), and the structure composed of thin and thick layers (Type C, large Mach number).

Description: In this paper, the scaling property of the inverse energy cascade and forward enstrophy cascade of the vorticity filed ω( x , y ) in two-dimensional (2D) turbulence is analyzed. This is accomplished by applying a Hilbert-based technique, namely Hilbert-Huang transform, to a vorticity field obtained from a 8192 2 grid-points direct numerical simulation of the 2D turbulence with a forcing scale k f = 100 and an Ekman friction. The measured joint probability density function p ( C , k ) of mode C i ( x ) of the vorticity ω and instantaneous wavenumber k ( x ) is separated by the forcing scale k f into two parts, which correspond to the inverse energy cascade and the forward enstrophy cascade. It is found that all conditional probability density function p ( C | k ) at given wavenumber k has an exponential tail. In the inverse energy cascade, the shape of p ( C | k ) does collapse with each other, indicating a nonintermittent cascade. The measured scaling exponent ζ ω I ( q ) is linear with the statistical order q , i.e., ζ ω I ( q ) = − q / 3 , confirming the nonintermittent cascade process. In the forward enstrophy cascade, the core part of p ( C | k ) is changing with wavenumber k , indicating an intermittent forward cascade. The measured scaling exponent ζ ω F ( q ) is nonlinear with q and can be described very well by a log-Poisson fitting: ζ ω F ( q ) = 1 3 q + 0.45 1 − 0 . 43 q . However, the extracted vorticity scaling exponents ζ ω ( q ) for both inverse energy cascade and forward enstrophy cascade are not consistent with Kraichnan's theory prediction. New theory for the vorticity field in 2D turbulence is required to interpret the observed scaling behavior.

Description: [1]  We analyse 1980-2010 ground displacements, to discern similarities or differences between Campi Flegrei (CF) inflations and deflations and highlight possible anomalies in particular areas. We show that the deformation pattern can be decomposed into two stationary (constant over time, except for a mere scaling factor) parts; both of them are satisfied by simple deformation sources. A quasi-horizontal elongated crack (oriented NW to SE, and embedded in an elastic layered half-space at a depth of about 3600 m) satisfies large-scale deformation. All source parameters but potency (volume change) are constant over time. Residual deformation is confined to the area of the Solfatara fumarolic field and satisfied by a small spheroid located at about 1900 m in depth. Again, all source parameters but potency are constant over time. The histories of the two sources are somewhat similar but not equal, supporting the existence of a genuine local deformation source at Solfatara against the emergence of a mere distortion of large-scale deformation. Although reality is probably much more complex, our simple model explains 1980-2010 CF deformation within ground-displacement data errors, and is consistent with Solfatara geochemical conceptual models, fumarolic geochemical data, and seismic attenuation imaging of CF. The observation that the CF deformation pattern can be decomposed into two stationary parts is hardly compatible with several recent works which proposed multiple sources with different features acting in different periods, fluid injections implying ample changes of large-scale deformation pattern over time, complex spatial and temporal patterns of distributions of volumetric sources.

Description: We report experimental observations of the controlled deformation of a dielectric liquid jet subjected to a local high-voltage electrostatic field in the direction normal to the jet. The jet deforms to the shape of an elliptic cylinder upon application of a normal electrostatic field. As the applied electric field strength is increased, the elliptic cylindrical jet deforms permanently into a flat sheet, and eventually breaks-up into droplets. We interpret this observation—the stretch of the jet is in the normal direction to the applied electric field—qualitatively using the Taylor-Melcher leaky dielectric theory, and develop a simple scaling model that predicts the critical electric field strength for the jet-to-sheet transition. Our model shows a good agreement with experimental results, and has a form that is consistent with the classical drop deformation criterion in the Taylor-Melcher theory. Finally, we statistically analyze the resultant droplets from sheet breakup, and find that increasing the applied electric field strength improves droplet uniformity and reduces droplet size.

Description: Normal impingement of a single droplet on a thin liquid film is investigated numerically solving the axisymmetric Navier-Stokes equations. Gravity and viscosity are taken into account whereas compressibility effects are neglected. Two phases are tracked by means of volume of fluid method and adaptive mesh refinement is used to increase accuracy of the interface. Numerical results are validated both qualitatively and quantitatively using experimental measurements. Effects of gas density, gas viscosity, and film thickness on the crown behavior are studied. Influence of droplet deviation from spherical shape on the crown behavior is investigated. It is shown that increasing the gas density leads to reduction of crown radius evolution rate, while gas viscosity does not affect the rate of crown radius evolution. Development rate of crown height decreases by increasing the gas density. Reynolds number and splashing regime can change the effect of gas viscosity on the crown height evolution. Deviation of droplet from sphere can change behavior of crown completely as result of change in droplet mass center position. Difference between numerical results and experimental ones is justified using different droplet shapes.

Description: We present an analytical study of peak mode isotachophoresis (ITP), and provide closed form solutions for sample distribution and electric field, as well as for leading-, trailing-, and counter-ion concentration profiles. Importantly, the solution we present is valid not only for the case of fully ionized species, but also for systems of weak electrolytes which better represent real buffer systems and for multivalent analytes such as proteins and DNA. The model reveals two major scales which govern the electric field and buffer distributions, and an additional length scale governing analyte distribution. Using well-controlled experiments, and numerical simulations, we verify and validate the model and highlight its key merits as well as its limitations. We demonstrate the use of the model for determining the peak concentration of focused sample based on known buffer and analyte properties, and show it differs significantly from commonly used approximations based on the interface width alone. We further apply our model for studying reactions between multiple species having different effective mobilities yet co-focused at a single ITP interface. We find a closed form expression for an effective-on rate which depends on reactants distributions, and derive the conditions for optimizing such reactions. Interestingly, the model reveals that maximum reaction rate is not necessarily obtained when the concentration profiles of the reacting species perfectly overlap. In addition to the exact solutions, we derive throughout several closed form engineering approximations which are based on elementary functions and are simple to implement, yet maintain the interplay between the important scales. Both the exact and approximate solutions provide insight into sample focusing and can be used to design and optimize ITP-based assays.

Description: A novel waveform modified from the standard-sinusoidal function is adopted to enhance the virtual aeroshaping effect of the synthetic jets positioned at the front stagnation point of a circular cylinder. The waveform is characterized by a control parameter, namely, the suction duty cycle factor k , which is the ratio of the time duration of the suction cycle to that of the blowing cycle. The strength of the synthetic jet vortex pair is enhanced by increasing the suction duty cycle factor. The periodic closed envelope forms upstream of the circular cylinder for k ≤ 1.00, while the quasi-steady open envelope forms for k ≥ 2.00, acting the virtual aeroshaping effect. As a result, both the statistical characteristics and the vortex dynamics of the near-wake flow field change with the suction duty cycle factor. The recirculation region downstream of the circular cylinder becomes smaller or even disappears, and thus, the drag coefficient over the circular cylinder is reduced by increasing the suction duty cycle factor to k ≥ 1.00. The statistical mean and fluctuating velocities show corresponding changes in the near wake with the different wake patterns. For k ≤ 0.50, the wake vortex shows the antisymmetric shedding mode which is similar with the natural case. For 1.00 ≤ k ≤ 2.00, the wake vortex shows the bistable state mode, where vortex sheds with symmetric or antisymmetric mode; the antisymmetric shedding mode dominates the global flow field for k = 1.00, while it is the symmetric shedding mode that dominates the flow field for k = 2.00. For k = 4.00, it shows the antisymmetric shedding mode with a shorter vortex formation length than the natural case. The above findings indicate that the virtual aeroshaping effect of the synthetic jets can be enhanced by increasing the suction duty cycle factor so as to increase the momentum coefficient while keeping other control parameters unchanged, providing us another way for effective flow control.

Description: The dynamics of vapor-liquid interface are important because interfacial instability determines bubble growth, detachment frequency, waiting time, shape of bubbles, and the interrelationship between bubble formation sites. In this study, a detailed numerical simulation has been performed to understand the transition in bubble release pattern and multimode bubble formation in saturated pool boiling. The interfaces drop down alternatively at the nodes and antinodes of the wavelengths dictated by Rayleigh-Taylor instability and Taylor-Helmholtz instability. Due to higher degrees of superheat, vapor jets emanate from nodes and antinodes. An attempt has been made to predict the maximum and minimum heat fluxes during saturated pool boiling.

Description: We present a combination of experiment, theory, and modelling on laminar mixing at large Péclet number. The flow is produced by oscillating electromagnetic forces in a thin electrolytic fluid layer, leading to oscillating dipoles, quadrupoles, octopoles, and disordered flows. The numerical simulations are based on the Diffusive Strip Method (DSM) which was recently introduced (P. Meunier and E. Villermaux, “The diffusive strip method for scalar mixing in two-dimensions,” J. Fluid Mech.662, 134–172 (2010)) to solve the advection-diffusion problem by combining Lagrangian techniques and theoretical modelling of the diffusion. Numerical simulations obtained with the DSM are in reasonable agreement with quantitative dye visualization experiments of the scalar fields. A theoretical model based on log-normal Probability Density Functions (PDFs) of stretching factors, characteristic of homogeneous turbulence in the Batchelor regime, allows to predict the PDFs of scalar in agreement with numerical and experimental results. This model also indicates that the PDFs of scalar are asymptotically close to log-normal at late stages, except for the large concentration levels which correspond to low stretching factors.

Description: Janus droplets are compound droplets that consist of two adhering drops of different fluids that are suspended in a third fluid. We use the Shan-Chen lattice Boltzmann method for multicomponent mixtures to simulate Janus droplets at rest and in shear. In this simulation model, interfacial tensions are not known a priori from the model parameters and must be determined using numerical experiments. We show that interfacial tensions obtained with the Young-Laplace law are consistent with those measured from the equilibrium geometry. The regimes of adhering, separated, and engulfing droplets were explored. Two different adhesion geometries were considered for two-dimensional simulations of Janus droplets in shear. The first geometry resembles two adhering circles with small overlap. In the second geometry, the two halves are semicircular. For both geometries, the rotation rate of the droplet depends on its orientation. The width of the periodic simulation domain also affects the rotation rate of both droplet types up to an aspect ratio of 6:1 (width:height). While the droplets with the first geometry oscillated about the middle of the domain, the droplets of the second geometry did not translate while rotating. A four-pole vortex structure inside droplets of the second geometry was found. These simulations of single Janus droplets reveal complex behaviour that implies a rich range of possibilities for the rheology of Janus emulsions.

Description: [1]  Detailed three-dimensional (3-D) P - and S -wave attenuation ( Qp and Qs ) models of the crust and upper mantle under the entire Northeast Japan (Tohoku) arc from the Japan Trench to the Japan Sea coast are determined, for the first time, using a large number of high-quality t * data measured precisely from P - and S -wave spectra of local earthquakes. The suboceanic earthquakes used in this work are relocated precisely using sP depth phases. Our results reveal a prominent landward dipping high- Q zone representing the subducting Pacific slab, a landward dipping intermediate to high Q zone in the mantle wedge between the Pacific coast and the volcanic front, and significant low- Q anomalies in the crust and mantle wedge between the volcanic front and the Japan Sea coast. Prominent high- Q patches surrounded by low- Q anomalies are revealed in the interplate megathrust zone under the Tohoku forearc where the great 2011 Tohoku-oki earthquake ( Mw 9.0) occurred. The high- Q patches in the megathrust zone generally exhibit large coseismic slips of megathrust earthquakes and large slip deficit on the plate interface. We think that these high- Q patches represent asperities in the megathrust zone, whereas the low- Q anomalies reflect weakly coupled areas. We also find that the hypocenters of the 2011 Tohoku-oki earthquake and its large foreshock ( Mw 7.3) and two large megathrust aftershocks ( Mw 7.4, 7.7) are located in areas where Qp , Qs and Qp / Qs change abruptly. These results suggest that structural heterogeneities in the megathrust zone control the interplate seismic coupling and the nucleation of megathrust earthquakes.

Description: Understanding the physics of water evaporation from saline porous media is important in many natural and engineering applications such as durability of building materials and preservation of monuments, water quality, and mineral-fluid interactions. We applied synchrotron x-ray micro-tomography to investigate the pore-scale dynamics of dissolved salt distribution in a three dimensional drying saline porous media using a cylindrical plastic column (15 mm in height and 8 mm in diameter) packed with sand particles saturated with CaI 2 solution (5% concentration by mass) with a spatial and temporal resolution of 12 μ m and 30 min, respectively. Every time the drying sand column was set to be imaged, two different images were recorded using distinct synchrotron x-rays energies immediately above and below the K-edge value of Iodine. Taking the difference between pixel gray values enabled us to delineate the spatial and temporal distribution of CaI 2 concentration at pore scale. Results indicate that during early stages of evaporation, air preferentially invades large pores at the surface while finer pores remain saturated and connected to the wet zone at bottom via capillary-induced liquid flow acting as evaporating spots. Consequently, the salt concentration increases preferentially in finer pores where evaporation occurs. Higher salt concentration was observed close to the evaporating surface indicating a convection-driven process. The obtained salt profiles were used to evaluate the numerical solution of the convection-diffusion equation (CDE). Results show that the macro-scale CDE could capture the overall trend of the measured salt profiles but fail to produce the exact slope of the profiles. Our results shed new insight on the physics of salt transport and its complex dynamics in drying porous media and establish synchrotron x-ray tomography as an effective tool to investigate the dynamics of salt transport in porous media at high spatial and temporal resolution.

Description: The behaviour of low Reynolds number, non-Boussinesq fountains from four different nozzle geometries (one circular and three rectangular nozzles) are studied. High speed laser schlieren imaging is used to study the fountain behaviour (frequency and penetration height). Bi-orthogonal decomposition and dynamic mode decomposition (DMD) are used to understand the unsteady characteristics of fountains. The flow regimes of fountains are classified as steady, flapping, and flapping-bobbing type. The DMD technique successfully separates the bobbing oscillation from the combined flapping-bobbing oscillation of the fountain. The frequency of the flapping oscillation, and the frequency of the bobbing oscillation in the flapping-bobbing regime scales as St h Fr h = C 1 and S t h F r h 2 = C 2 , respectively, where the characteristic length scale is the smallest dimension ( h ) of the nozzle. The mean steady state penetration heights ( Z s / h ) of “forced” low Reynolds number non-Boussinesq fountains are independent of nozzle shape (circular and rectangular), and scales linearly with the Froude number.

Description: The large-scale properties of self-similar unstably stratified homogeneous (USH) turbulence are investigated using an eddy-damped quasi-normal markovianized approximation of the nonlinear term. This analysis shows that a special role is played by the wave vectors contained in the equatorial plane, i.e., the plane perpendicular to gravity. It is indeed in this plane that turbulent spectra reach their maxima and evolve linearly from their initial condition when their initial infrared exponent is smaller than 4. At other angles, this property is not satisfied and turbulent spectra eventually undergo an evolution dominated by nonlinear backscattering processes. The self-similar evolution of USH turbulence is also shown to be related to the properties of large scales. In particular, the asymptotic growth rate of the mixing length depends on the initial infrared exponent in the equatorial plane. Besides, the self-similar asymptotic values of the concentration and velocity correlations also depend on the properties of large scales. This allows to derive relations between the correlations and the growth rate parameter.

Description: [1]  We detected 32,078 very small, local microearthquakes (average M L  = -1) during a 9-month deployment of five ocean bottom seismometers (OBS) on the periphery of the Trans-Atlantic Geotraverse (TAG) active mound. Seismicity rates were constant without any mainshock-aftershock behavior at ~243 events per day at the beginning of the experiment, 128 events per day after an instrument failed, and 97 events per day at the end of the experiment when whale calls increased background noise levels. The microearthquake seismograms are characterized by durations of 〈1 second and most have single-phase P -wave arrivals (i.e., no S -arrivals). We accurately located 6,207 of the earthquakes, with hypocenters clustered within a narrow depth interval from ~50-125 mbsf on the south and west flanks of the deposit. We model the microearthquakes as reaction-driven fracturing events caused by anhydrite deposition in the secondary circulation system of the hydrothermal mound, and show that under reasonable modeling assumptions an average event represents a volume increase of 31-58 cm 3 , yielding an annual (seismogenic) anhydrite deposition rate of 27-51 m 3 .

Description: Secondary flow cells are commonly observed in straight laboratory channels, where they are often associated with duct corners. Here, we present velocity measurements acquired with an acoustic Doppler current profiler in a straight reach of the Seine river (France). We show that a remarkably regular series of stationary flow cells spans across the entire channel. They are arranged in pairs of counter-rotating vortices aligned with the primary flow. Their existence away from the river banks contradicts the usual interpretation of these secondary flow structures, which invokes the influence of boundaries. Based on these measurements, we use a depth-averaged model to evaluate the momentum transfer by these structures, and find that it is comparable with the classical turbulent transfer.

Description: [1]  Acoustic velocity measurements were conducted during triaxial deformation tests of silty clay and clayey silt core samples from the Nankai subduction zone (IODP Expeditions 315, 316 and 333). We provide a new set of sonic wave velocity data, continuously measured during pressure increase and subsequent axial deformation. A new processing method for the experimental data was developed using seismic time series analysis. The results show that the identification of first arrivals by manual trace-by-trace picking alone can be erroneous. During axial deformation compressional wave velocities (Vp) range between about 1300 - 2200 m/s, and shear wave velocities (Vs) range between about 150 - 800 m/s. Vp slightly increases with rising effective confining pressure and effective axial stress. Samples from the accretionary prism toe show the highest Vp, while those from the forearc slope sediments show somewhat lower Vp. The samples from the incoming plate are slightly richer in clay minerals, and have the lowest values for Vp. Vs increases with higher effective confining pressures and effective axial stress, irrespective of composition and tectonic setting. Shear and bulk moduli, calculated from sonic velocities are in the range between 0.2 and 1.3 GPa and 3.85 and 8.41 GPa, respectively. The elastic moduli of the samples from the accretionary prism toe and the footwall of the megasplay fault ranging between 1.50 and 3.98 GPa are higher compared to those from the hanging wall samples and the incoming plate samples ranging between 0.59 and 0.88 Gpa. Thus, they allow differentiation between normal and over-consolidated sediments. The data show that in a tectono-sedimentary environment of only subtle compositional differences, the acoustic properties can be used to differentiate between stronger (accretionary prism toe) and weaker (forearc slope, incoming plate) sediments. Especially the Vp/Vs ratio may be instrumental in detecting zones of low effective stress and thus high pore fluid pressure.

Description: [1]  Studies of the petrology, mineral chemistry and rock-magnetic properties of nine pyroxenite xenoliths from Hannuoba basalts, northern North China Craton, have been made to determine the magnetization signature of the continental lower crust. These pyroxenites are weakly magnetic with low average susceptibility ( χ ) and saturation isothermal remanent magnetization ( M rs ) of 39.59 × 10 -8  m 3  kg -1 and 12.05 × 10 -3 Am 2 kg -1 , respectively. The magnetic minerals are mainly magnetite, pyrrhotite and Fe-rich spinel, which significantly contribute to χ and natural remanent magnetization. Magnetite occurs as interstitial micro-crystals together with zeolite aggregates, indicating a secondary origin in a supergene environment. In contrast, pyrrhotite and Fe-rich spinel were formed prior to the xenoliths’ ascent to the surface, as evidenced by their dominant occurrence as tiny inclusions and thin exsolution lamellae in pyroxene. The Fe-rich spinel has ~ 50% mole-fraction of Fe 3 O 4 and corresponds to the strongest magnetization, and its coexistence with Mg-rich spinel implies a reheating event due to the underplating of basaltic magma. Besides, armalcolite and ilmenite were found in the reaction rims between xenoliths and the basalt, but they contribute little to the whole rock magnetization. However, these pyroxenite xenoliths would be non-magnetic at in situ depths, as well as peridotite and mafic granulite xenoliths derived from the crust-mantle transition zone (~ 32-42 km). Therefore, we suggest the limiting depth of magnetization at the boundary between weakly magnetic deep-seated (lower crust and upper mantle) xenoliths and strongly magnetic Archean granulite facies rocks (~ 32 km) in Hannuoba, northern North China Craton.

Description: Analysis of fluxes across the turbulent/non-turbulent interface (TNTI) of turbulent boundary layers is performed using data from two-dimensional particle image velocimetry (PIV) obtained at high Reynolds numbers. The interface is identified with an iso-surface of kinetic energy, and the rate of change of total kinetic energy ( K ) inside a control volume with the TNTI as a bounding surface is investigated. Features of the growth of the turbulent region into the non-turbulent region by molecular diffusion of K , viscous nibbling, are examined in detail, focussing on correlations between interface orientation, viscous stress tensor elements, and local fluid velocity. At the level of the ensemble (Reynolds) averaged Navier-Stokes equations (RANS), the total kinetic energy K is shown to evolve predominantly due to the turbulent advective fluxes occurring through an average surface which differs considerably from the local, corrugated, sharp interface. The analysis is generalized to a hierarchy of length-scales by spatial filtering of the data as used commonly in Large-Eddy-Simulation (LES) analysis. For the same overall entrainment rate of total kinetic energy, the theoretical analysis shows that the sum of resolved viscous and subgrid-scale advective flux must be independent of scale. Within the experimental limitations of the PIV data, the results agree with these trends, namely that as the filter scale increases, the viscous resolved fluxes decrease while the subgrid-scale advective fluxes increase and tend towards the RANS values at large filter sizes. However, a definitive conclusion can only be made with fully resolved three-dimensional data, over and beyond the large dynamic spatial range presented here. The qualitative trends from the measurement results provide evidence that large-scale transport due to the energy-containing eddies determines the overall rate of entrainment, while viscous effects at the smallest scales provide the physical mechanism ultimately responsible for entrainment. Data spanning over a decade in Reynolds number suggest that the fluxes (or the entrainment velocity) scale with the friction velocity (or equivalently the local turbulent fluctuating velocity), whereas Taylor microscale and boundary-layer thickness are the appropriate length scales at small and large filter sizes, respectively.

Description: [1]  The shallow velocity structure of the Lucky Strike segment of the Mid-Atlantic Ridge is investigated using seismic refraction and reflection techniques applied to downward continued multi-channel streamer data. We present a three-dimensional velocity model beneath the Lucky Strike Volcano with unprecedented spatial resolutions of a few hundred meters. These new constraints reveal large lateral variations in P-wave velocity structure beneath this feature. Throughout the study area, uppermost crustal velocities are significantly lower than those inferred from lower-resolution OBS studies, with the lowest values (1.8-2.2 km/s) found beneath the three central volcanic cones. Within the central volcano, distinct shallow units are mapped that likely represent a systematic process such as burial of older weathered surfaces. We infer that the entire upper part of the central volcano is young relative to the underlying median valley floor and that there has been little increase in the layer 2A velocities since emplacement. Layer 2A thins significantly across the axial valley bounding faults likely as the result of footwall uplift. The upper crustal velocities increase with age, on average, at a rate of ~0.875 km/s/Myr, similar to previous measurements at fast spreading ridges, suggesting hydrothermal sealing of small scale porosity is progressing at normal to enhanced rates.

Description: In a recent direct numerical simulation (DNS) study [P. K. Yeung and K. R. Sreenivasan, “ Spectrum of passive scalars of high molecular diffusivity in turbulent mixing,” J. Fluid Mech.716, R14 (2013)] with Schmidt number as low as 1/2048, we verified the essential physical content of the theory of Batchelor, Howells, and Townsend [“Small-scale variation of convected quantities like temperature in turbulent fluid. 2. The case of large conductivity,” J. Fluid Mech.5, 134 (1959)] for turbulent passive scalar fields with very strong diffusivity, decaying in the absence of any production mechanism. In particular, we confirmed the existence of the −17/3 power of the scalar spectral density in the so-called inertial-diffusive range. In the present paper, we consider the DNS of the same problem, but in the presence of a uniform mean gradient, which leads to the production of scalar fluctuations at (primarily) the large scales. For the parameters of the simulations, the presence of the mean gradient alters the physics of mixing fundamentally at low Peclet numbers. While the spectrum still follows a −17/3 power law in the inertial-diffusive range, the pre-factor is non-universal and depends on the magnitude of the mean scalar gradient. Spectral transfer is greatly reduced in comparison with those for moderately and weakly diffusive scalars, leading to several distinctive features such as the absence of dissipative anomaly and a new balance of terms in the spectral transfer equation for the scalar variance, differing from the case of zero gradient. We use the DNS results to present an alternative explanation for the observed scaling behavior, and discuss a few spectral characteristics in detail.

Description: A non-equilibrium wall-model based on unsteady 3D Reynolds-averaged Navier-Stokes (RANS) equations has been implemented in an unstructured mesh environment. The method is similar to that of the wall-model for structured mesh described by Wang and Moin [Phys. Fluids14, 2043–2051 (2002)], but is supplemented by a new dynamic eddy viscosity/conductivity model that corrects the effect of the resolved Reynolds stress (resolved turbulent heat flux) on the skin friction (wall heat flux). This correction is crucial in predicting the correct level of the skin friction. Unlike earlier models, this eddy viscosity/conductivity model does not have a stress-matching procedure or a tunable free parameter, and it shows consistent performance over a wide range of Reynolds numbers. The wall-model is validated against canonical (attached) transitional and fully turbulent flows at moderate to very high Reynolds numbers: a turbulent channel flow at Re τ = 2000, an H-type transitional boundary layer up to Re θ = 3300, and a high Reynolds number boundary layer at Re θ = 31 000. Application to a separated flow over a NACA4412 airfoil operating close to maximum lift is also considered to test the performance of the wall-model in complex non-equilibrium flows.

Description: Conventional shallow water theory successfully reproduces many key features of the Jovian atmosphere: a mixture of coherent vortices and stable, large-scale, zonal jets whose amplitude decreases with distance from the equator. However, both freely decaying and forced-dissipative simulations of the shallow water equations in Jovian parameter regimes invariably yield retrograde equatorial jets, while Jupiter itself has a strong prograde equatorial jet. Simulations by Scott and Polvani [“Equatorial superrotation in shallow atmospheres,” Geophys. Res. Lett.35, L24202 (2008)] have produced prograde equatorial jets through the addition of a model for radiative relaxation in the shallow water height equation. However, their model does not conserve mass or momentum in the active layer, and produces mid-latitude jets much weaker than the equatorial jet. We present the thermal shallow water equations as an alternative model for Jovian atmospheres. These equations permit horizontal variations in the thermodynamic properties of the fluid within the active layer. We incorporate a radiative relaxation term in the separate temperature equation, leaving the mass and momentum conservation equations untouched. Simulations of this model in the Jovian regime yield a strong prograde equatorial jet, and larger amplitude mid-latitude jets than the Scott and Polvani model. For both models, the slope of the non-zonal energy spectra is consistent with the classic Kolmogorov scaling, and the slope of the zonal energy spectra is consistent with the much steeper spectrum observed for Jupiter. We also perform simulations of the thermal shallow water equations for Neptunian parameter values, with a radiative relaxation time scale calculated for the same 25 mbar pressure level we used for Jupiter. These Neptunian simulations reproduce the broad, retrograde equatorial jet and prograde mid-latitude jets seen in observations. The much longer radiative time scale for the colder planet Neptune explains the transition from a prograde to a retrograde equatorial jet, while the broader jets are due to the deformation radius being a larger fraction of the planetary radius.

Description: The hydrodynamic interaction of two deformable vesicles in shear flow induces a net displacement, in most cases an increase of their distance in the transverse direction. The statistical average of these interactions leads to shear-induced diffusion in the suspension, both at the level of individual particles which experience a random walk made of successive interactions, and at the level of suspension where a nonlinear down-gradient diffusion takes place, an important ingredient in the structuring of suspension flows. We make an experimental and computational study of the interaction of a pair of lipid vesicles in shear flow by varying physical parameters, and investigate the decay of the net lateral displacement with the distance between the streamlines on which the vesicles are initially located. This decay and its dependency upon vesicle properties can be accounted for by a simple model based on the well established law for the lateral drift of a vesicle in the vicinity of a wall. In the semi-dilute regime, a determination of self-diffusion coefficients is presented.

Description: [1]  High quality streaming potential coupling coefficient measurements have been carried out using a newly designed cell with both a steady-state methodology and a new pressure transient approach. The pressure transient approach has shown itself to be particularly good at providing high quality streaming potential coefficient measurements as each transient increase or decrease allows thousands of measurements to be made at different pressures to which a good linear regression can be fitted. Nevertheless, the transient method can be up to five times as fast as the conventional measurement approaches because data from all flow rates are taken in the same transient measurement rather than separately. Test measurements have been made on samples of Berea and Boise sandstone as a function of salinity (approximately 18 salinities between 10-5 mol/dm3 and 2 mol/dm3). The data have also been inverted to obtain the zeta potential. The streaming potential coefficient becomes greater (more negative) for fluids with lower salinities, which is consistent with existing measurements. Our measurements are also consistent with the high salinity streaming potential coefficient measurements made by Vinogradov et al. (2010). Both the streaming potential coefficient and the zeta potential have also been modeled using the theoretical approach of Glover et al. (2012). This modeling allows the microstructural, electrochemical and fluid properties of the saturated rock to be taken into account in order to provide a relationship that is unique to each particular rock sample. In all cases, we found that the experimental data was a good match to the theoretical model.

Description: [1]  The present study, which is a follw-up of the JGR paper by Ji et al. (2013a), provides a new calibration for both seismic and fabric properties of antigorite serpentinites. Comparisons of the laboratory velocities of antigorite serpentinites measured at high pressures with crystallographic preferred orientation (CPO) data measured using electron backscatter diffraction (EBCD) techniques demonstrate that seismic anisotropy in high T serpentinite, which is essentially controlled by the antigorite c-axis fabric, is independent on the operating slip system, but strongly dependent on the regime and magnitude of finite strain experienced by the rock. Extrapolation of the experimental data with both pressure and temperature suggests that V p anisotropy decreases but shear wave splitting (Δ V s ) and V p / V s increase with increasing pressure in either cold or hot subduction zones. For a cold, steeply subducting slab, antigorite is most likely deformed by nearly coaxial flattening or trench-parallel movements, forming trench-parallel seismic anisotropy. For a hot, shallowly subducting slab, however, antigorite is most likely deformed by simple shear or transpression. Trench-normal seismic anisotropy can be observed when the subducting dip angle is smaller than 30°. The geophysical characteristics of the Tibetan Plateau such as strong heterogeneity in V p , V s and attenuation, shear wave splitting and electric conductivity may be explained by the presence of strongly deformed serpentinites in lithospheric shear zones reactivated along former suture zones between amalgamated blocks, hydrated zones of subducting lithospheric mantle, and the crust-mantle boundary if the temperature is below 700 °C in the region of interest.

Description: [1]  We explore the concept of maximum possible earthquake magnitude, M , in a region represented by an earthquake catalog from the viewpoint of statistical testing. For this aim, we assume that earthquake magnitudes are independent events that follow a doubly-truncated Gutenberg-Richter distribution and focus on the upper truncation M . In earlier work, it has been shown that the value of M cannot be well constrained from earthquake catalogs alone. However, for two hypothesized values M and M ′, alternative statistical tests may address the question: Which value is more consistent with the data? In other words: Is it possible to reject a magnitude within reasonable errors, i.e. the error of the first and the error of the second kind? The results for realistic settings indicate that either the error of the first kind or the error of the second kind is intolerably large. We conclude that it is essentially impossible to infer M in terms of alternative testing with sufficient confidence from an earthquake catalog alone, even in regions like Japan with excellent data availability. These findings are also valid for frequency-magnitude distributions with different tail behavior, e.g. exponential tapering. Finally, we emphasize that different data may only be useful to provide additional constraints for M , if they do not correlate with the earthquake catalog, i.e. if they have not been recorded in the same observational period. In particular, longterm geological assessments might be suitable to reduce the errors, while GPS measurements provide overall the same information as the catalogs.

Description: [1]  We simulated the ascent of bubbly magma in a volcanic conduit by slow decompression experiments using syrup foam as a magma analogue. During decompression, some large voids appear in the foam. The expansion of one void deep in the foam leads to another void expansion, and the void expansion then propagates upward. The void expansion finally reaches the surface of the foam to originate outgassing. The velocity of the upward propagation of void expansions is essentially the same as the rupturing velocity of the bubble film, suggesting that the rupture of films separating each void propagates upward to create the pathway for outgassing. The calculated apparent permeability of decompressed foam can become higher than that measured for natural pumices/scoriae. The upward propagation of film ruptures thus allows for efficient outgassing. This may also appear as the mechanism for energetic gas emissions originating at a depth, such as Strombolian eruptions.

Description: The magnetohydrodynamic Richtmyer-Meshkov instability is investigated for the case where the initial magnetic field is unperturbed and aligned with the mean interface location. For this initial condition, the magnetic field lines penetrate the perturbed density interface, forbidding a tangential velocity jump and therefore the presence of a vortex sheet. Through simulation, we find that the vorticity distribution present on the interface immediately after the shock acceleration breaks up into waves traveling parallel and anti-parallel to the magnetic field, which transport the vorticity. The interference of these waves as they propagate causes the perturbation amplitude of the interface to oscillate in time. This interface behavior is accurately predicted over a broad range of parameters by an incompressible linearized model derived presently by solving the corresponding impulse driven, linearized initial value problem. Our use of an equilibrium initial condition results in interface motion produced solely by the impulsive acceleration. Nonlinear compressible simulations are used to investigate the behavior of the transverse field magnetohydrodynamic Richtmyer-Meshkov instability, and the performance of the incompressible model, over a range of shock strengths, magnetic field strengths, perturbation amplitudes and Atwood numbers.

Description: We investigate experimentally the statistical properties of bedload transport induced by a steady, uniform, and laminar flow. We focus chiefly on lateral transport. The analysis is restricted to experiments where the flow-induced shear stress is just above the threshold for sediment transport. We find that, in this regime, the concentration of moving particles is low enough to neglect interactions between themselves. We can therefore represent bedload as a thin layer of independent walkers travelling over the bed surface. In addition to their downstream motion, the particles show significant fluctuations of their cross-stream velocity, likely due to the roughness of the underlying sediment bed. This causes particles to disperse laterally. Based on thousands of individual trajectories, we show that this lateral spreading is the manifestation of a random walk. The experiments are entirely consistent with Fickian diffusion.

Description: We study the behavior of heavy inertial particles in the flow field of two like-signed vortices. In a frame co-rotating with the two vortices, we find that stable fixed points exist for these heavy inertial particles; these stable frame-fixed points exist only for particle Stokes number St 〈 St cr . We estimate St cr and compare this with direct numerical simulations, and find that the addition of viscosity increases the St cr slightly. We find that the rate at which particles fall into the fixed points increases until the fixed points disappear at St = St cr . These frame-fixed points are between fixed points and limit cycles in character.

Description: [1]  Geodetic surveys now provide detailed time series maps of anthropogenic land subsidence and uplift due to injection and withdrawal of pore fluids from the subsurface. A coupled poroelastic model allows the integration of geodetic and hydraulic data in a joint inversion and has therefore the potential to improve the characterization of the subsurface and our ability to monitor pore pressure evolution. We formulate a Bayesian inverse problem to infer the lateral permeability variation in an aquifer from geodetic and hydraulic data, and from prior information. We compute the maximum a posteriori (MAP) estimate of the posterior permeability distribution, as well as a Gaussian approximation of the posterior. Computing the MAP estimate requires the solution of a large-scale minimization problem subject to the poroelastic equations, for which we propose an efficient Newton-conjugate gradient optimization algorithm. The covariance matrix of the Gaussian approximation of the posterior is given by the inverse Hessian of the log posterior, which we construct by exploiting low rank properties of the data misfit Hessian. First and second derivatives are computed using adjoints of the time dependent poroelastic equations, allowing us to fully exploit transient data. Using three increasingly complex model problems, we find the following general properties of poroelastic inversions: Augmenting standard hydraulic well data by surface deformation data improves the aquifer characterization. Surface deformation contributes the most in shallow aquifers, but provides useful information even for the characterization of aquifers down to 1 km. In general, it is more difficult to infer high permeability regions, and their characterization requires frequent measurement to resolve the associated short response time scales. In horizontal aquifers, the vertical component of the surface deformation provides a smoothed image of the pressure distribution in the aquifer. Provided that the mechanical properties are known, coupled poroelastic inversion is therefore a promising approach to detect flow barriers and to monitor pore pressure evolution.

Description: The paper reports a new phenomenon—vortex flows in isothermal magnetic fluids in the vicinity of the localized source of magnetic field (magnetized iron sphere) induced by the drift of drop-like aggregates. Although the observed magnetic precipitation of drop-like aggregates resembles an ordinary rainfall in the Earth atmosphere, its origin and nature are quite different. In magnetic fluids this “rain” is induced by the non-uniform magnetic field and occurs at the scale of 1 mm, not at the scale of several kilometers as in the Earth atmosphere. The reason of this phenomenon is that the applied magnetic field initiates phase transition of “gas-liquid” type which is accompanied by formation of condensed phase represented by drop-like aggregates with the characteristic dimension of about tens of micrometers elongated along the field lines. Inhomogeneous spatial distribution of drop-like aggregates leads to deviation of the ponderomotive force, which is responsible for the formation of vortex flows in the fluid. The “rain” is the primary reason for the vortex flows and it lasts until all magnetic particles capable of condensing into drop-like aggregates precipitate at the surface of the condensation core (iron sphere). Thus, vortex flows induced by drop-like aggregate magnetophoresis represent one variant of “gas-liquid” phase transition. Hydrodynamic flows intensify mass transfer in vicinity of magnetic condensation core and considerably speed it up.

Description: [1]  Numerical simulation experiments give insight into the evolving energy partitioning during high strain torsion experiments of calcite. Our numerical experiments are designed to derive a generic macroscopic grain size sensitive flow law capable of describing the full evolution from the transient regime to steady state. The transient regime is crucial for understanding the importance of microstructural processes that may lead to strain localization phenomena in deforming materials. This is particularly important in geological and geodynamic applications where the phenomenon of strain localization happens outside the time frame that can be observed under controlled laboratory conditions. Our method is based on an extension of the paleowattmeter approach to the transient regime. We add an empirical hardening law using the Ramberg-Osgood approximation and assess the experiments by an evolution test function of stored over dissipated energy (lambda factor). Parameter studies of, strain hardening, dislocation creep parameter, strain rates, temperature and lambda factor as well as mesh sensitivity are presented to explore the sensitivity of the newly derived transient/steady state flow law on key quantities. Our analysis can be seen as one of the first steps in a hybrid computational-laboratory-field modeling workflow. The analysis could be improved through independent verifications by thermographic analysis in physical laboratory experiments to independently assess lambda factor evolution under laboratory conditions.

Description: [1]  Despite its importance, the temporal and spatial evolution of continental dynamic topography is poorly known. Australia's isolation from active plate boundaries and its rapid northward motion within a hotspot reference frame make it a useful place to investigate the interplay between mantle convection, topography and drainage. Offshore, dynamic topography is relatively well constrained and can be accounted for by Australia's translation over the mantle's convective circulation. To build a databaseof onshore constraints, we have analyzed an inventory of longitudinal river profiles, which is sensitive to uplift rate history. Using independently constrained erosional parameters, we determine uplift rates by minimizing the misfit between observed and calculated river profiles. Resultant fits are excellent and calculated uplift histories match independent geologic constraints. We infer that western and central Australia underwent regional uplift during the last 50 Myr and that theEastern Highlands have been uplifted in two stages. The first stage from 120–80 Ma, coincided with rifting along the eastern margin and its existence is supported by thermochronological measurements. A second stage occurred at 80–10 Ma, formed the Great Escarpment, and coincided with Cenozoic volcanism. The relationship between topography, gravity anomalies, and shear wave tomographic models suggest that regional elevation is supported by temperature anomalies within the lithosphere's thermal boundary layer. Morphology and stratigraphy of the Eastern Highlands imply that these anomalies have been coupled to the base of the plate during Australia's northward motion over the last 70 Myr.

Description: Compressive sampling is well-known to be a useful tool used to resolve the energetic content of signals that admit a sparse representation. The broadband temporal spectrum acquired from point measurements in wall-bounded turbulence has precluded the prior use of compressive sampling in this kind of flow, however it is shown here that the frequency content of flow fields that have been Fourier transformed in the homogeneous spatial (wall-parallel) directions is approximately sparse, giving rise to a compact representation of the velocity field. As such, compressive sampling is an ideal tool for reducing the amount of information required to approximate the velocity field. Further, success of the compressive sampling approach provides strong evidence that this representation is both physically meaningful and indicative of special properties of wall turbulence. Another advantage of compressive sampling over periodic sampling becomes evident at high Reynolds numbers, since the number of samples required to resolve a given bandwidth with compressive sampling scales as the logarithm of the dynamically significant bandwidth instead of linearly for periodic sampling. The combination of the Fourier decomposition in the wall-parallel directions, the approximate sparsity in frequency, and empirical bounds on the convection velocity leads to a compact representation of an otherwise broadband distribution of energy in the space defined by streamwise and spanwise wavenumber, frequency, and wall-normal location. The data storage requirements for reconstruction of the full field using compressive sampling are shown to be significantly less than for periodic sampling, in which the Nyquist criterion limits the maximum frequency that can be resolved. Conversely, compressive sampling maximizes the frequency range that can be recovered if the number of samples is limited, resolving frequencies up to several times higher than the mean sampling rate. It is proposed that the approximate sparsity in frequency and the corresponding structure in the spatial domain can be exploited to design simulation schemes for canonical wall turbulence with significantly reduced computational expense compared with current techniques.

Description: This paper presents extensive acoustic measurements on jets impinging on surfaces of various surface roughness values. Besides surface roughness, the effects of nozzle-to-plate spacing distance and nozzle pressure ratio are also investigated. Turbulent mixing noise and tonal noise are explained using far-field wall-jet velocity and impingement region temperature fields. The results demonstrate that roughness of the impingement plate widens the staging region of impingement noise. In general, high speed jet impinging on a rough plate generates less noise compared to a smooth plate. When tones are removed from the spectra, it is found that acoustic power monotonically decreases with increasing surface roughness. Thermal imaging in the stagnation region indicates that whenever tones are present, the temperature at the stagnation region is high. Further, sound pressure directivity pattern of impingement noise is constructed by superimposing a wall-jet and a free jet in the appropriate orientations.

Description: We investigate the elasticity of an isolated, threefold junction of soap films (Plateau border), which displays static undulations when liquid rapidly flows into it. By analyzing the shape of the Plateau border (thickness R and transverse displacement) as a function of the liquid flow rate Q , we show experimentally and theoretically that the elasticity of the Plateau border is dominated by the bending of the soap films pulling on the Plateau border. In this asymptotic regime, the undulation wavelength obeys the scaling law ∼ Q 2   R −2 and the decay length ∼ Q 2   R −4 .

Description: We present an efficient numerical method for earthquake cycle simulations that employs a finite difference discretization of the off-fault material to accommodate spatially variable elastic properties. The method is developed for the two-dimensional antiplane shear problem of a vertical strike-slip fault with rate-and-state friction. We compare earthquake cycles in a homogeneous half-space with those in which the upper portion of the fault cuts through a sedimentary basin. In both cases, we assume velocity-weakening behavior over the full seismogenic depth, even in the basin, to isolate the influence of elastic heterogeneity. In a homogeneous half-space, events rupturing the full seismogenic depth occur periodically. Event sequences are more complex in basin models, with one or several sub-basin events confined to the lower section of the fault followed by a much larger, surface-rupturing event that breaks through the basin. This phenomenology emerges only for sufficiently compliant and deepbasins. Predicted surface velocities are essentially identical before sub-basin events and surface-rupturing events, suggesting that geodetic observations would not be useful in predicting the rupture mode. The alternating sequence of sub-basin andsurface-rupturing events would complicate interpretation of paleoseismic data. Our results also offer one potential explanation for the shallow slip deficit that has been observed in many recent earthquakes, namely, that these events, which lack appreciable surface slip, are simply one style of rupture. Subsequent events on these faults might be larger, with slip extending all the way to the surface. The 1940 M w 7.0 and 1979 M w 6.5 Imperial Valley events might be considered as examples of these two rupture styles.

Description: The 290-km-long, Nayband strike-slip fault bounds the western margin of the Lut block and cuts across a region thought to have been quiescent during the last few millennia. Cl-36 cosmic ray exposure (CRE) and optically stimulated luminescence (OSL) dating of cumulative geomorphic offsets are used to derive the long-term slip rate. The measured offsets at two sites along the fault range between 9 ± 1 m and 195 ± 15 m with ages from 6.8 ± 0.6 ka to ∼ 100 ka, yielding minimum and maximum bounds of late Pleistocene and Holocene slip rates of 1.08 and 2.45 mm yr -1 , respectively. This moderate slip rate of 1.8 ± 0.7 mm yr -1 , averaged over several earthquake cycles, is compared to the paleoseismic record retrieved from the first trench excavated across the fault. Combining the paleoseismic evidence with 18 OSL ages obtained from this trench site demonstrates the occurrence of at least four large (M w  ∼ 7) earthquakes during the last 17.4 ± 1.3 ka and of two older earthquakes, one before ∼ 23 ka and another before 70 ± 5 ka. The exposed sediment succession also indicates a significant gap at the end of MIS-2 and the beginning of MIS-1. The age of the most recent regional incision is accurately bracketed between 6.1 ka and 7.4 ka. Sediments from the last ∼ 7 ka contain evidence of the three younger earthquakes. Interestingly, the penultimate and antepenultimate events occurred between 6.5 ± 0.4 ka and 6.7 ± 0.4 ka within a time interval lasting at most 1 ka whereas the most recent earthquake occurred within the last millennium. Such an irregular earthquake occurrence suggests the seismic behavior of the Nayband fault is not strictly time dependent but possibly related to clustering. From this and taking into account the occurrence of the most recent earthquake within the last 800 years, the imminence of an earthquake along the Nayband fault cannot be discarded. Although the most recent surface-rupturing event seems to have occurred after AD 1200, this event went unnoticed in the historical records. This provides a marked illustration of the incompleteness of the historical seismic catalogs in Central Iran, challenging any assessment of regional seismic hazard without appropriate geologic and geochronological information. Large and infrequent earthquakes are characteristic of the seismic behavior of the slow-slipping strike-slip faults slicing Central and Eastern Iran. Also, the slip rates summed across Central and Eastern Iran from the Iran Plateau up to the Afghan lowlands appear in agreement with the most recent GPS data.

Description: Yellowstone National Park (YNP) displays numerous and extensive hydrothermal features. Although hydrothermal alteration in YNP has been extensively studied, the volume, geometry, and type of rock alteration at depth remain poorly constrained. In this study, we use high-resolution airborne and ground magnetic surveys and measurements of remanent and induced magnetization of field and drill-hole samples to provide constraints on the geometry of hydrothermal alteration within the subsurface of three thermal areas in YNP (Firehole River, Smoke Jumper Hot Springs, and Norris Geyser Basin). We observe that hydrothermal zones from both liquid- and vapor-dominated systems coincide with magnetic lows observed in aeromagnetic surveys and with a decrease of the amplitude of short-wavelength anomalies seen in ground magnetic surveys. This suggests strong demagnetization of both the shallow and deep substratum within these areas associated with the removal or transformation of magnetic minerals by hydrothermal alteration processes. Such demagnetization is confirmed by measurements of rock samples from hydrothermal areas which display significantly decreased total magnetization. Measurements of rock samples from drill holes also suggest that the degree of demagnetization and therefore hydrothermal alteration of the substratum varies dramatically over short distances both vertically and horizontally. A pronounced negative anomaly is observed over the Lone Star Geyser and suggests a significant demagnetization of the substratum associated with areas displaying large-scale fluid flow. We use the damped short-wavelength signal over ground magnetic profiles to map hydrothermal alteration in the shallow substratum. Maximum values of the magnetic anomaly gradient measured along ground magnetic profiles are also used to estimate the thickness of a non-magnetic layer that is assumed to cover the volcanic unaltered substratum. Finally, the magnetic lows observed over ground and airborne magnetic surveys are used to invert the distribution of magnetization using two types of simplifying assumptions. First, the substratum magnetization is inverted assuming it is constant in the vertical direction and second, the topography of the base of a superficial non-magnetic layer is inverted assuming the underlying substratum has a constant magnetization. Both the magnetic gradient analysis and the topography inversion show that significant demagnetization occurs over a thickness of at least a few hundred meters in hydrothermal areas at YNP. We also show that the maximum degree or maximum thickness of demagnetization correlates closely with the location of hydrothermal activity and mapped alteration. Our results suggest that areas of hydrothermal alteration are composed of substrata with different degrees of alteration, with significant alteration occurring within narrow zones that may represent major conduits for hydrothermal fluid flow.

Description: The inverse problem of determining the flow at the Earth's Core Mantle Boundary according to an outer core magnetic field and secular variation model has been investigated through a Bayesian formalism. To circumvent the issue arising from the truncated nature of the available fields, we combined two modeling methods. In the first step, we applied a filter on the magnetic field to isolate its large scales by reducing the energy contained in its small scales, we then derived the dynamical equation, referred as filtered frozen flux equation, describing the spatio-temporal evolution of the filtered part of the field. In the second step, we proposed a statistical parametrization of the filtered magnetic field in order to account for both its remaining unresolved scales and its large scale uncertainties. These two modeling techniques were then included in the Bayesian formulation of the inverse problem. To explore the complex posterior distribution of the velocity field resulting from this development, we numerically implemented an algorithm based on Markov Chain Monte Carlo methods. After evaluating our approach on synthetic data and comparing it to previously introduced methods, we applied it to a magnetic field model derived from satellite data for the single epoch 2005.0. We could confirm the existence of specific features already observed in previous studies. In particular we retrieved the planetary-scale eccentric gyre characteristic of flow evaluated under the compressible quasi-geostrophy assumption although this hypothesis was not considered in our study. In addition, through the sampling of the velocity field posterior distribution, we could evaluate the reliability, at any spatial location and at any scale, of the flow we calculated. The flow uncertainties we determined are nevertheless conditioned by the choice of the prior constraints we applied to the velocity field.

Description: The case of a turbulent round jet impinging perpendicularly onto a rotating, heated disc is investigated, in order to understand the mechanisms at the origin of the influence of rotation on the radial wall jet and associated heat transfer. The present study is based on the complementary use of an analysis of the orders of magnitude of the terms of the mean momentum and Reynolds stress transport equations, available experiments, and dedicated Reynolds-averaged Navier–Stokes computations with refined turbulence models. The Reynolds number Re j = 14 500, the orifice-to-plate distance H = 5 D , where D is the jet-orifice diameter, and the four rotation rates were chosen to match the experiments of Minagawa and Obi [“Development of turbulent impinging jet on a rotating disk,” Int. J. Heat Fluid Flow25, 759–766 (2004)] and comparisons are made with the Nusselt number distribution measured by Popiel and Boguslawski [“Local heat transfer from a rotating disk in an impinging round jet,” J. Heat Transfer108, 357–364 (1986)], at a higher Reynolds number. The overestimation of turbulent mixing in the free-jet before the impact on the disk is detrimental to the prediction of the impingement region, in particular of the Nusselt number close to the symmetry axis, but the self-similar wall jet developing along the disk is correctly reproduced by the models. The analysis, experiments, and computations show that the rotational effect do not directly affect the outer layer, but only the inner layer of the wall jet. A noteworthy consequence is that entrainment at the outer edge of the wall jet is insensitive to rotation, which explains the dependence of the wall-jet thickness on the inverse of the non-dimensional rotation rate, observed in the experiments and the Reynolds stress model computations, but not reproduced by the eddy-viscosity models, due to the algebraic dependence to the mean flow. The analysis makes moreover possible the identification of a scenario for the appearance of rotational effects when the rotation rate is gradually increased. For weak rotation rates, the rotation-induced boundary layer appears but does not break the self-similar solution observed for the case without rotation. For intermediate rotation rates, the production of the azimuthal Reynolds stress becomes much stronger than other components, leading to a complete modification of the turbulence anisotropy which is reproduced only by Reynolds stress models. For strong rotation rates, centrifugal effects dominate, leading to an acceleration and thinning of the jet, and consequently an increase of turbulent production and heat transfer, reproduced by all the turbulence models.

Description: Microfluidic channels are powerful means of control of minute volumes such as droplets. These droplets are usually conveyed at will in an externally imposed flow which follows the geometry of the micro-channel. It has recently been pointed out by Dangla et al. [“Trapping microfluidic drops in wells of surface energy,” Phys. Rev. Lett.107(12), 124501 (2011)] that the motion of transported droplets may also be stopped in the flow, when they are anchored to grooves which are etched in the channels top wall. This feature of the channel geometry explores a direction that is usually uniform in microfluidics. Herein, this anchoring effect exploiting the three spatial directions is studied combining a depth averaged fluid description and a geometrical model that accounts for the shape of the droplet in the anchor. First, the presented method is shown to enable the capture and release droplets in numerical simulations. Second, this tool is used in a numerical investigation of the physical mechanisms at play in the capture of the droplet: a localized reduced Laplace pressure jump is found on its interface when the droplet penetrates the groove. This modified boundary condition helps the droplet cope with the linear pressure drop in the surrounding fluid. Held on the anchor the droplet deforms and stretches in the flow. The combination of these ingredients leads to recover the scaling law for the critical capillary number at which the droplets exit the anchors C a ★ ∝ h 2 / R 2 where h is the channel height and R the droplet undeformed radius.

Description: We present a numerical study of nanosecond pulsed dielectric barrier discharge (DBD) actuator operating in quiescent air at atmospheric condition. Our study concentrates on plasma discharge induced fluid dynamics and on exploration of parametric space of interest for voltage pulse in an attempt to shed some light into elucidation of the mechanisms whereby the generated shock wave propagates through and affects the external flow. Specifically, a one-dimensional, self-similar, local ionization kinetic model recently developed to predict key parameters of nanosecond pulsed plasma discharge is coupled with the compressible Navier-Stokes equations possibly for the first time. Within the considered range of parameters of the plasma model which is justified for the modeling of surface nanosecond pulsed discharge at atmospheric pressure, our coupled method is able to provide satisfactory prediction of the shock structure generated by the actuator for comparison with experiment, not only in the qualitative shock wave shape but also in quantitative shock front displacement. We provide a comprehensive analysis of the gas heating, shock wave initiation and evolution processes. For example, the characteristic time of the rapid localized heating responsible for shock wave generation, which is yet to be quantified experimentally, is found to be ∼350 ns. We conduct a parametric investigation by varying the peak voltage from 10 kV to 50 kV and rise time from 5 ns to 150 ns. The pressure wave whose behavior is found to be dominated by input voltage amplitude, introduces highly transient, localized disturbance to the quiescent air. In addition, the vortex induced by the shock passage is relatively weak. The interplay of the induced flows by a few successive plasma discharges operating at continuous mode does not appear to be significant, especially at low voltage amplitude.

Description: In this paper, we investigate the decay of incompressible homogeneous isotropic turbulence in a variable viscosity fluid. The viscosity coefficient is assumed to depend linearly on a scalar, representing either a temperature or a concentration, and obeying a simple advection-diffusion equation. At high Reynolds numbers, Direct Numerical Simulations (DNS) allow us to confirm the validity of Taylor's postulate that the dissipation is independent from the viscosity and its fluctuations. At low Reynolds numbers, we report the presence of extra energy at small scales due to these variable viscosity effects. This implies that the turbulent kinetic energy decreases less rapidly as a function of time in variable viscosity fluids. In order to explain this phenomenon and quantify its importance on the turbulent flow, we propose a statistical approach based on an eddy-damped quasi-normal Markovian (EDQNM) spectral closure which takes into account the nonlinearity introduced by variable viscosity. It is shown that this latter additional term is of constant sign in the energy spectrum equation and reduces the dissipation of the flow as observed. Also, by assuming the dominance of distant interactions between wave numbers, we can propose a simple formula expressing that variable viscosity effects lead to an effective reduction of the mean viscosity proportional to the variance of viscosity fluctuations.

Description: Liquid droplets flowing through a rectangular microfluidic channel develop a vortical flow field due to the presence of shear forces from the surrounding fluid. In this paper, we present an experimental and computational study of droplet velocities and internal flow patterns in a rectangular pressure-driven flow for droplet diameters ranging from 0.1 to 2 times the channel height. Our study shows excellent agreement with asymptotic predictions of droplet and interfacial velocities for infinitesimally small droplets. As the droplet diameter nears the size of the channel height, the droplet velocity slows significantly, and the changing external flow field causes a qualitative change in the location of internal vortices. This behavior is relevant for future studies of mass transfer in microfluidic devices.

Description: Laboratory experiments are performed to examine the formation of a crater in sediment by an impinging vertical turbulent jet. Light attenuation and a “depositometer,” which records conductivity through the bed from an array of electrodes, are used to measure the crater depth as a function of space and time. The onset of crater formation and deepening is best characterized in terms of the Rouse number, Rs (proportional to the particle settling speed divided by the centerline jet speed), rather than Shields number, Sh (proportional to the stress divided by the particle weight per unit area). The critical Rouse number, Rs c , is found to increase with the particle Reynolds number, Re p , as a power law with exponent 0.45 ± 0.03 for Re p ranging between 0.6 and 160. For smaller Rs, the crater is observed to deepen at a near-constant speed, while the crater radius remains constant. Bedload transport, measured in terms of the crater deepening speed, is determined to increase as Re p times the difference between Rs c and Rs.

Description: We report the experimental studies of the statistical and scaling properties of the fully developed turbulent regime in von Karman swirling flow between counter-rotating disks with and without blades using the only global measurements of the spatially averaged torque Γ and pressure p fluctuations in water and water-sugar solutions of different viscosities in the same cell geometry. We show that for all fluids under investigation probability distribution functions (PDFs) of the torque fluctuations δΓ/Γ rms are Gaussian in both the laminar and turbulent regimes and for the both types of the stirrers. On the contrary, PDFs of the pressure fluctuations change from Gaussian in the laminar regime into the skewed shape with the exponential tails toward low-pressure events for both the entrainment methods. Both the friction coefficient C f and normalized rms of the pressure fluctuations C p are independent of Re in the fully developed turbulent regime for all fluids under study and found in a good quantitative agreement with the previous results. We also observe that the internal flow variables such as the normalized torque Γ ¯ / V p r m s versus the “internal” Reynolds number Re rms = ( p rms /ρ) 1/2 R ρ/η instead of the global variables C f , C p versus Re show sharp transition into the well developed turbulent regime. We find that the scaling exponents of the fundamental characteristics based only on Γ and p measurements in the range of fully developed turbulent flow, namely, the integral, Taylor, and Kolmogorov dissipation lengths, as well as the Taylor-based Reynolds number R λ , are in rather fair agreement with the predictions. We would like to emphasize that scaling of the main turbulent parameters R λ , λ, η d obtained via the global variables is a very non-trivial result. It is not obvious that measurements based on the global quantities will provide the predicted scaling relations. The result on such scaling obtained previously strongly disagrees with the scaling predictions. Indeed, both Γ ¯ and p rms are averaged over the cell volume as well as all spatial scales, whereas the swirling flow is neither isotropic nor homogeneous. So the global variables being averaged over all spatial scales get contributions from the scales larger and smaller than those from the inertial range of scales. And finally, the normalized characteristic frequencies f p / f rot found in both the torque and pressure frequency power spectra in the fully developed turbulent regime have close values, are independent of Re , and associated with either the rotation or oscillation frequency of the main vortex.

Description: The Longitudinal Valley Fault (LVF) in Eastern Taiwan is a high slip rate fault (about 5 cm/yr), which exhibits both seismic and aseismic slip. Deformation of anthropogenic features shows that aseismic creep accounts for a significant fraction of faultslip near the surface, whereas a fraction of the slip is also seismic, since this fault has produced large earthquakes with five M w  〉 6.8 events in 1951 and 2003. In this study, we analyze a dense set of geodetic and seismological data around the LVF, including campaign-mode Global Positionnig System(GPS) measurements, time series of daily solutions for continuous GPS stations (cGPS), leveling data, and accelerometric records of the 2003 Chenkung earthquake. To enhance the spatial resolutionprovided by these data, we complement them with Interferometric Synthetic Aperture Radar (InSAR) measurements produced from a series of Advanced Land Observing Satellite (ALOS) images processed using a persistent scatterer (PS) technique. The combined dataset covers the entire LVF and spans the period from 1992 to 2010. We invert this data to infer the temporal evolution of fault slip at depth using the Principal Component Analysis-based Inversion Method (PCAIM). This technique allows the joint inversion of diverse data, taking the advantage of the spatial resolution given by the InSAR measurements and the temporal resolution afforded by the cGPS data. We find that (1) seismic slip during the 2003 Chengkung earthquake occurred on a fault patch whichhad remained partially locked in the interseismic period; (2) the seismic rupture propagated partially into a zone of shallow aseismic interseismic creep but failed to reach the surface; (3) that aseismic afterslip occurred around the area that ruptured seismically. We find consistency between geodetic and seismological constraints on the partitioning between seismic and aseismic creep. About 80-90% of slip on the LVF in the 0-26 km, seismogenic depth range is actually aseismic. We infer that the clay-rich Lichi Mélange is the key factor promoting aseismic creep at shallow depth.

Description: Numerical computations are presented to study the effect of soluble surfactant on the deformation and breakup of an axisymmetric drop or bubble stretched by an imposed linear strain flow in a viscous fluid. At the high values of bulk Peclet number Pe in typical fluid-surfactant systems, there is a thin transition layer near the interface in which the surfactant concentration varies rapidly. The large surfactant gradients are resolved using a fast and accurate “hybrid” numerical method that incorporates a separate, singular perturbation analysis of the dynamics in the transition layer into a full numerical solution of the free boundary problem. The method is used to investigate the dependence of drop deformation on parameters that characterize surfactant solubility. We also compute resolved examples of tipstreaming, and investigate its dependence on parameters such as flow rate and bulk surfactant concentration.

Description: Multi-scale analysis is widely adopted in turbulence research for studying flow structures corresponding to specific length scales in the Kolmogorov spectrum. In the present work, a new methodology based on novel optimization techniques for scale decomposition is introduced, which leads to a bandpass filter with prescribed properties. With this filter, we can efficiently perform scale decomposition using Fourier transform directly while adequately suppressing Gibbs ringing artifacts. Both 2D and 3D scale decomposition results are presented, together with qualitative and quantitative analysis. The comparison with existing multi-scale analysis technique is conducted to verify the effectiveness of our method. Validation of this decomposition technique is demonstrated both qualitatively and quantitatively. The advantage of the proposed methodology enables a precise specification of continuous length scales while preserving the original structures. These unique features of the proposed methodology may provide future insights into the evolution of turbulent flow structures.

Description: Direct numerical simulations (DNS) are conducted for a Mach 2.75 turbulent boundary layer interacting with an impinging shock at three different shock incidence angles. The accuracies of DNS calculations are established by checking the convergence of flow statistics for various grids, by comparing the generated results with those in the literature and also by the balance of contributing terms in the turbulent kinetic energy equation. Instantaneous flow visualizations show the significant effect of shock on turbulence structure in the shock-boundary layer interaction zone and also in the flow downstream of the interaction region. The separation bubbles exhibit highly unsteady and three-dimensional behavior and are larger for stronger shocks but the maximum probability of flow separation is found to be independent of the shock strength. The differences between Reynolds- and Favre-averaged quantities are also observed to be small and largely independent of the shock intensity. The turbulent kinetic energy is amplified across the shock, mainly by the production term in the turbulent kinetic energy equation. The amplification of enstrophy across the shock zone is found to be due to the vortex stretching term in the enstrophy transport equation. A detailed examination of the terms in the turbulent kinetic equation shows a strong coupling between the mean and turbulent fields in the interaction region with energy being continuously exchanged from one field to another. However, the compressibility-related terms in the transport equations for turbulent kinetic energy and enstrophy are found to be small for the simulated flows.

Description: Lotus-type porous metals are a promising alternative for compact heat transfer applications. In lotus-type porous fins, jet impingement and transverse mixing play important roles for heat transfer: jets emerging from the pores impinge on the following fin and enhance heat transfer performance, while the transverse fluid motion advects heat away from the fin surface. By means of magnetic resonance imaging we have performed mean flow and scalar transport measurements through scaled-up replicas of two kinds of lotus-type porous fins: one with a deterministic hole pattern and staggered alignment, and one with a random hole pattern, but the same porosity and mean pore diameter. The choice of geometric parameters (fin spacing, thickness, porosity, and hole diameter) is based on previous thermal studies. The Reynolds number based on the mean pore diameter and inner velocity ranges from 80 to 3800. The measurements show that in the random hole pattern the jet characteristic length scale is substantially larger with respect to the staggered hole pattern. The random geometry also produces long coherent vortices aligned with the streamwise direction, which improves the transverse mixing. The random hole distribution causes the time mean streamlines to meander in a random-walk manner, and the diffusivity coefficient associated to the mechanical dispersion (which is nominally zero in the staggered hole configuration) is several times larger than the fluid molecular diffusivity at the higher Reynolds numbers. From the trends in maximum streamwise velocity, streamwise vorticity, and mechanical diffusivity, it is inferred that the flow undergoes a transition to an unsteady/turbulent regime around Reynolds number 300. This is supported by the measurements of concentration of an isokinetic non-buoyant plume of scalar injected upstream of the stack of fins. The total scalar diffusivity for the fully turbulent regime is found to be 22 times larger than the molecular diffusivity, but only 6 times higher than the mechanical diffusivity, indicating that the latter plays a significant role for heat transfer and mixing.

Description: We performed numerical calculations of compaction in aggregates of spherical grains, using Lehner and Leroy's (2004) constitutive model of pressure solution at grain contacts (LL). That model is founded on a local definition of the thermodynamic driving force and leads to a fully coupled formulation of elastic deformation, dissolution, and diffusive transport along the grain boundaries. The initial geometry of the aggregate was generated by random packing of spheres with a small standard deviation of the diameters. During the simulations, isostatic loading was applied. The elastic displacements at the contacts were calculated according to Digby's (1981) non-linear contact force model and deformation by dissolution was evaluated using the LL formulation. The aggregate strain and porosity were tracked as a function of time for fixed temperature, applied effective pressure, and grain size. We also monitored values of the average and standard deviation of total load at each contact, the coordination number for packing, and the statistics of the contact dimensions. Because the simulations explicitly exclude processes such as fracturing, plastic flow, and transport owing to surface curvature, they can be used to test the influence of relative changes in the kinetics of dissolution and diffusion processes caused by contact growth and packing re-arrangements. We found that the simulated strain data could be empirically fitted by two successive power laws of the form, ε x  ∝  t ξ , where ξ was equal to 1 at very early times, but dropped to as low as 0.3 at longer times. The apparent sensitivity of strain rate to stress found in the simulations was much lower than predicted from constitutive laws that assume a single dominant process driven by average macroscopic loads. Likewise, the apparent activation enthalpy obtained from the simulated data was intermediate between that assumed for dissolution and diffusion, and, further, tended to decrease with time. These results are similar to the experimental observations of Visser et al.’s (2012), who used an aggregate geometry and physical conditions closely resembling the present numerical simulations.

Description: To understand the energy release process that operates at the end of the mainshock rupture and start of the aftershock activity, we propose an inversion method that uses continuous high-frequency seismogram envelopes of the mainshock and early aftershocks (i.e., events that occur at short times after the mainshock). In our approach, the aftershock sequence is regarded as a continuous energy release process, rather than a discrete time series of events. To correct for the contribution of coda wave energy excited by multiple scattering, we use the theoretical envelope synthesized on the basis of the radiative transfer theory as a Green's function. The site amplification factors are corrected considering the conservation of energy flux and using the coda normalization method. The inverted temporal energy release rate for the 2008 M W 6.9 Iwate-Miyagi Nairiku earthquake, Japan, decays following t -1.1 , at the lapse time t of 40-900 s after the mainshock origin time. This exponent of the decay rate is similar to the p -value of the modified Omori law. The amount of estimated energy release is consistent with that calculated from the magnitude listed in the aftershock catalog. Although the uncertainty is large, the location of large energy release at the lapse times of 40-900 s approximately overlaps to that of the aftershocks, which surrounds the large energy release area during the mainshock faulting. The maxima of the energy release rate normalized by the average decay rate distributes following a power-law, similar to the Gutenberg-Richter law.

Description: In this article, we discuss flows in shallow, stratified horizontal layers of two immiscible fluids. The top layer is an electrolyte which is electromagnetically driven and the bottom layer is a dielectric fluid. Using a quasi-two-dimensional approximation, which assumes a horizontal flow whose direction is independent of the vertical coordinate, we derive a generalized two-dimensional vorticity equation describing the evolution of the horizontal flow. Also, we derive an expression for the vertical profile of the horizontal velocity field. Measuring the horizontal velocity fields at the electrolyte-air and electrolyte-dielectric interfaces using particle image velocimetry, we validate the theoretical predictions of the horizontal velocity and its vertical profile for steady as well as for freely decaying Kolmogorov-like flows. Our analysis shows that by increasing the viscosity of the electrolyte relative to that of the dielectric, one may significantly improve the uniformity of the flow in the electrolyte, yielding excellent agreement between the analytical predictions and the experimental measurements.

Description: We investigate the direct enstrophy cascade of two-dimensional decaying turbulence in a flowing soap film channel. We use a coarse-graining approach that allows us to resolve the nonlinear dynamics and scale-coupling simultaneously in space and in scale. From our data, we verify an exact relation due to Eyink [“Local energy flux and the refined similarity hypothesis,” J. Stat. Phys.78, 335–351 (1995); Eyink “Exact results on scaling exponents in the 2D enstrophy cascade,” Phys. Rev. Lett.74, 3800–3803 (1995)] between traditional 3rd-order structure function and the enstrophy flux obtained by coarse-graining. We also present experimental evidence that enstrophy cascades to smaller (larger) scales with a 60% (40%) probability, in support of theoretical predictions by Merilees and Warn [“On energy and enstrophy exchanges in two-dimensional non-divergent flow ,” J. Fluid Mech.69, 625–630 (1975)] which appear to be valid in our flow owing to the ergodic nature of turbulence. We conjecture that their kinematic arguments break down in quasi-laminar 2D flows. We find some support for these ideas by using an Eulerian coherent structure identification technique, which allows us to determine the effect of flow topology on the enstrophy cascade. A key finding is that “centers” are inefficient at transferring enstrophy between scales, in contrast to “saddle” regions which transfer enstrophy to small scales with high efficiency.

Description: We studied the terminal velocity of a packed array of bubbles, a bubble cluster, rising in different fluids: a Newtonian fluid, an elastic fluid with nearly constant viscosity (Boger fluid), and a viscoelastic fluid with a shear dependent viscosity, for small but finite Reynolds numbers (1 × 10 −4 〈 Re 〈 4). In all three cases, the cluster velocity increased with the total volume, following the same trend as single bubbles. For the case of clusters in elastic fluids, interestingly, the so-called velocity discontinuity was not observed, unlike the single bubble case. In addition to the absence of jump velocity, the clusters did not show the typical teardrop shape of large bubbles in viscoelastic fluids and the strength of the negative wake is much weaker than the one observed behind single bubbles. Dimensional analysis of the volume-velocity plots allowed us to show that, while the equivalent diameter (obtained from the total cluster volume) is the appropriate length to determine buoyancy forces and characteristic shear rates, the individual bubble size is the appropriate scale to account for surface forces.

Description: Determining the melt distribution in oceanic crust at mid-ocean ridges is critical to understanding how magma is transported and emplaced in the crust. Seafloor compliance —deformation under ocean wave forcing — is primarily sensitive to regions of low shear velocity in the crust, making it a useful tool to probe melt distribution. Analysis of compliance data collected at East Pacific Rise between 9°-10ºN through 3D numerical modeling reveals strong along-axis variations in the lower crustal shear velocities, as well as temporal variation of crustal shear velocity near 9°48’N between measurements spanning 8 years. Compliance measured across the rise axis at 9°48’N and 9°33’N suggest a deep crustal low velocity zone (DLZ) beneath the ridge axis, with a low Vs/Vp ratio consistent with melt in low aspect ratio cracks or sills. Changes in compliance measured at 9°48’N between years 1999 and 2007 suggest that the melt fraction in the axial crust decreased during this interval, perhaps following the 2005-2006 seafloor eruption. This temporal variability provides direct evidence for short-term variations of the magmatic system at a fast-spreading ridge.

Description: About 150 km of high-resolution, seismic-reflection (CHIRP) profiles (ca. 20 m penetration) were collected in Mono Lake in order to define the uppermost sedimentary architecture of the basin, which has been heavily impacted by recent volcanic, tectonic, and climatic processes. The study also provides important background for ongoing efforts to obtain paleoenvironmental records from sediment cores in the lake. The history of four seismic-stratigraphic units in the upper 20 m of section are inferred from the data, and the interpretations are generally consistent with previous interpretations of lake history for the past 2000 years, including a major lowstand at 1941 m. No shorelines below the previously documented major lowstand at 1941 m were found. A relatively steep slope segment, whose toe is at about 1918 m, and which occurs on the southern and western margins of the deep basin of the lake, is interpreted as the relict foreset slope of deposition from prograding western tributaries. This topography is unconformably overlain by a unit of interbedded tephra and lake sediments of variable lithology, which contains tephra of the North Mono (600–625 cal yr BP) eruption in its upper part. The tephra-rich unit is overlain by a mostly massive mudflow deposit that is locally more than 18 m thick and that is distributed in a radial pattern around Paoha Island. The evidence suggests that, within the past few hundred years, rapid uplift of Paoha Island through thick, pre-existing lake deposits led to widespread slope failures, which created a terrain of disrupted, intact blocks near the island, and a thick, fluid mudflow beyond. As is common in mudflows, the mudflow moved up the depositional slope of the lake floor, terminating against the pre-existing slopes, likely in multiple surges. Since about 1700 CE, fine-grained, well-laminated sediments have accumulated in the deep parts of the lake at anomalously rapid rates, probably driven by continued rapid, small-scale shedding of sediment from Paoha Island.

Description: Surface measurements of shear wave splitting patterns are widely used to infer the mantle circulations around subducting slabs; however, the relation between mantle flow and seismic anisotropy is still elusive. Finite strain is a direct measurement of time-dependent deformation, and has been proposed as a proxy for the crystal preferred orientation (CPO) of mantle minerals. We have conducted a series of numerical models to systematically investigate the mantle flow, finite strain, olivine CPO and SKS wave splitting in oceanic subduction zones with variable slab width. They demonstrate that the preferred orientations of olivine a -axes generally agree with the long (extensional) axes of the finite strain ellipsoid (FSE), even in these very complex mantle flow fields; however, neither the a -axis nor the FSE axes necessarily aligns with the instantaneous mantle velocity vector. We identify two domains with distinct deformation mechanisms in the central sub-plate mantle, where simple shear induced by plate advance dominates at shallow depths and produces trench-normal fast splitting, while pure shear induced by slab rollback dominates the deeper mantle and results in trench-parallel fast splitting. The SKS splitting patterns are thus dependent on the competing effects of these two mechanisms, and also on the subduction partition ratio γ  =  X p /X t : trench-parallel when γ  〈 1, and trench-normal when γ  〉 1. In addition, variable mantle deformation mechanisms and SKS splitting patterns are observed in the mantle wedge and around the slab edges, which may aid in the general interpretation of seismic anisotropy observations in natural subduction zones.

Description: Gravity changes occurring during the initial stage of the 2011–2012 El Hierro submarine eruption are interpreted in terms of the pre-eruptive signatures during the episode of unrest. Continuous gravity measurements were made at two sites on the island using the relative spring gravimeter LCR gPhone-054. On September 15, 2011, an observed gravity decrease of 45 μGal, associated with the southward migration of seismic epicenters, is consistent with a lateral magma migration occurred beneath the volcanic edifice, an apparently clear precursor of the eruption that took place 25 days later on October 10, 2011. High-frequency gravity signals also appeared on October 6–11, 2011, point to an interaction between a magmatic intrusion and the ocean floor was occurring. These important gravity changes, with amplitudes varying from 10 to −90 μGal, during the first three days following the onset of the eruption are consistent with the northward migration of the eruptive focus along an active eruptive fissure. An apparent correlation of gravity variations with body tide vertical strain was also noted, which could indicate that concurrent tidal triggering occurred during the initial stage of the eruption.

Description: In this paper we study the behavior of a fluid-saturated fault under shear, based on the assumption that the material inside exhibits rate- and temperature-dependent frictional behavior. A creeping fault of this type can produce excess heat due to shearheating, reaching temperatures which are high enough to trigger endothermic chemical reactions. We focus on fluid-release reactions and incorporate excess pore pressure generation and porosity variations due to the chemical effects (a process called chemical pressurization). We provide the mathematical formulation for coupled thermo-hydro-chemo-mechanical processes and study the influence of the frictional, hydraulic and chemical properties of the material, along with the boundary conditions of the problem on the behavior of the fault. Regimes of stable-frictional sliding and pressurization emerge, and the conditions for the appearance of periodic creep-to-pressurization instabilities are then derived. The model thus extends the classical mechanical stick- slip instabilities by identifying chemical pressurization as the process governing the slip phase. The different stability regimes identified match the geological observations about subduction zones. The model presented was specifically tested in the Episodic Tremor and Slip sequence of the Cascadia megathrust, reproducing the displacement data available from the GPS network installed. Through this process we identify that the slow slip events in Cascadia could be due to the in-situ dehydration of serpentinite minerals. During this process the fluid pressures increase to sub-lithostatic values and lead to the weakening of the creeping slab.

Description: This study investigates the influence of the substrate surface roughness on the emplacement mechanisms of pyroclastic flows. We carried out laboratory experiments on gravitational flows generated from the release of initially fluidized or non-fluidized columns of fine particles (diameter d  = 0.08 mm) in a horizontal channel. The roughness of the channel base was uniform in each experiment, created by gluing particles of diameter d 0  = 0.08 to 6 mm to the base. Other things being equal, the flow runout distance increased with the channel base roughness ( d 0 ) to a maximum of about twice that of flows on a smooth substrate when d 0  = 1.5-3 mm, before decreasing moderately at higher roughness values of d 0  = 6 mm. Long runout originated mainly during the late stages of emplacement as flow deceleration was very much reduced at high substrate roughness. This was caused by (partial) autofluidization due to an upward flux of air escaping from the substrate interstices in which flow particles settled. Autofluidization was evidenced by high pore fluid pressure measurements at the base of initially non-fluidized flows, and also by reduced flow runout when the interstices were initially partially filled so that less air was available. Furthermore, the runout distance of flows of large particles ( d  = 0.35 mm), which could not be fluidized by the ascending air flux, was independent of the substrate roughness. This study suggests that autofluidization caused by air escape from the interstices of a rough substrate is one important mechanism to explain the common long runout distance of pyroclastic flows even on sub-horizontal topographies.

Description: The dynamics and multiple-cycle evolution of the incompressible flow induced by a moving piston through the open valve of a motored piston-cylinder assembly was investigated using direct numerical simulation. A spectral element solver, adapted for moving geometries using an Arbitrary Lagrange/Eulerian formulation, was employed. Eight cycles were simulated and the ensemble- and azimuthally-averaged data were found to be in good agreement with experimentally determined means and fluctuations at all measured points and times. During the first half of the intake stroke the flow field is dominated by the dynamics of the incoming jet and the vortex rings it creates. With decreasing piston speed a large central ring becomes the dominant flow feature until the top dead center. The flow field at the end of the previous cycle is found to have a dominant effect on the jet breakup and the vortex ring dynamics below the valve and on the observed significant cyclic variations. Based on statistical averaging, the evolution of the turbulent flow field during the first half of the intake stroke is dominated by the jet breakup process leading to a strongly anisotropic behavior. In the second part of the intake stroke, the decrease of the incoming jet velocity results in a more isotropic behavior.

Description: This paper presents the linear stability analysis of an interface between air and an insulating liquid subjected to a perpendicular electric field, in the presence of unipolar injection of charge. Depending on the characteristics of the liquid and the depth of the liquid layer two different instability thresholds may be found. One of them is characterized by a wavelength of the order of the liquid layer thickness and corresponds to the well-known volume instability of a liquid layer subjected to charge injection. The other one is characterized by a wavelength some ten times the liquid layer thickness and corresponds to the so-called rose-window instability, an instability associated to the balance of surface stresses.

Description: An alternative framework for parameterizing stably stratified shear-flow turbulence is presented. Using dimensional analysis, four non-dimensional parameters of interest are identified that consider the independent effects of stratification, shear, viscosity, and scalar diffusivity. In the interest of geophysical applications, the problem is further simplified by considering only high Reynolds number flow. This leads to a two-dimensional parameter space based on a buoyancy strength parameter (i.e., an inverse Froude number) and a shear strength parameter. Consideration for the gradient Richardson number allows the space to be divided into an unforced regime, a shear-dominated regime, and a buoyancy-dominated regime. On this basis, a large database of direct numerical simulation and laboratory data from various sources is evaluated. Of particular interest is the observed length scale of overturning. Overturns are found to scale with k 1/2 / N in the buoyancy-dominated regime, k 1/2 / S in the shear-dominated regime, and k 3/2 /ε in the unforced regime, where k , N , S , and ε are the turbulent kinetic energy, buoyancy frequency, mean shear rate, and turbulent kinetic energy dissipation rate, respectively. Implications for estimates of diapycnal mixing in the ocean are discussed and a new parameterization for eddy diffusivity is presented.

Description: We quantify the strength of the waves and their impact on the energy cascade in rotating turbulence by studying the wave number and frequency energy spectrum, and the time correlation functions of individual Fourier modes in numerical simulations in three dimensions in periodic boxes. From the spectrum, we find that a significant fraction of the energy is concentrated in modes with wave frequency ω ≈ 0, even when the external forcing injects no energy directly into these modes. However, for modes for which the period of the inertial waves τ ω is faster than the turnover time τ NL , a significant fraction of the remaining energy is concentrated in the modes that satisfy the dispersion relation of the waves. No evidence of accumulation of energy in the modes with τ ω = τ NL is observed, unlike what critical balance arguments predict. From the time correlation functions, we find that for modes with τ ω 〈 τ sw (with τ sw the sweeping time) the dominant decorrelation time is the wave period, and that these modes also show a slower modulation on the timescale τ NL as assumed in wave turbulence theories. The rest of the modes are decorrelated with the sweeping time, including the very energetic modes with ω ≈ 0.

Description: We propose a new mechanism for bubble nucleation triggered by the rubbing of solid surfaces immersed in a liquid, in which the fluid molecules squeezed between the solids are released with high kinetic energy into the bulk of the liquid, resulting in the nucleation of a vapor bubble. Molecular dynamics simulations with a superheated Lennard-Jones fluid are used to evidence this mechanism. Nucleation is observed at the release of the squeezed molecules, for squeezing pressures above a threshold value and for all the relative velocities between the solids that we investigate. We show that the total kinetic energy of the released molecules for a single release event is proportional to the number of molecules released, which depends on the squeezing pressure, but is independent of the velocity.

Description: The Reynolds number scaling of flow topology in the eigenframe of the strain-rate tensor is investigated for wall-bounded flows, which is motivated by earlier works showing that such topologies appear to be qualitatively universal across turbulent flows. The databases used in the current study are from direct numerical simulations (DNS) of fully developed turbulent channel flow (TCF) up to friction Reynolds number Re τ ≈ 1500, and a spatially developing, zero-pressure-gradient turbulent boundary layer (TBL) up to Re θ ≈ 4300 ( Re τ ≈ 1400). It is found that for TCF and TBL at different Reynolds numbers, the averaged flow patterns in the local strain-rate eigenframe appear the same consisting of a pair of co-rotating vortices embedded in a finite-size shear layer. It is found that the core of the shear layer associated with the intense vorticity region scales on the Kolmogorov length scale, while the overall height of the shear layer and the distance between the vortices scale well with the Taylor micro scale. Moreover, the Taylor micro scale collapses the height of the shear layer in the direction of the vorticity stretching. The outer region of the averaged flow patterns approximately scales with the macro scale, which indicates that the flow patterns outside of the shear layer mainly are determined by large scales. The strength of the shear layer in terms of the peak tangential velocity appears to scale with a mixture of the Kolmogorov velocity and root-mean-square of the streamwise velocity scaling. A quantitative universality in the reported shear layers is observed across both wall-bounded flows for locations above the buffer region.